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Title: Geological Observations on South America
Author: Charles Darwin
Release Date: June 17, 2001 [eBook #3620]
[Most recently updated: December 12, 2021]
Language: English
*** START OF THE PROJECT GUTENBERG EBOOK GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA ***
Geological Observations on South America
By Charles Darwin
CONTENTS
EDITORIAL NOTE.
DETAILED TABLE OF CONTENTS.
GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA
CRITICAL INTRODUCTION.
CHAPTER I. ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.
CHAPTER II. ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.
CHAPTER III. ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL DEPOSITS.
CHAPTER IV. ON THE FORMATIONS OF THE PAMPAS.
CHAPTER V. ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.
CHAPTER VI. PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.
CHAPTER VII. CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.
CHAPTER VIII. NORTHERN CHILE.—CONCLUSION.
INDEX
EDITORIAL NOTE.
Although in some respects more technical in their subjects and style
than Darwin’s “Journal,” the books here reprinted will never lose their
value and interest for the originality of the observations they
contain. Many parts of them are admirably adapted for giving an insight
into problems regarding the structure and changes of the earth’s
surface, and in fact they form a charming introduction to physical
geology and physiography in their application to special domains. The
books themselves cannot be obtained for many times the price of the
present volume, and both the general reader, who desires to know more
of Darwin’s work, and the student of geology, who naturally wishes to
know how a master mind reasoned on most important geological subjects,
will be glad of the opportunity of possessing them in a convenient and
cheap form.
The three introductions, which my friend Professor Judd has kindly
furnished, give critical and historical information which makes this
edition of special value.
G.T.B.
DETAILED TABLE OF CONTENTS.
CRITICAL INTRODUCTION.
CHAPTER I.
ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.
Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and
Pumice-pebbles.—Step-formed plains of Patagonia, with upraised
shells.—Terrace-bounded valley of Santa Cruz, formerly a
sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth of
the elevated area.—Equability of the movements, as shown by the similar
heights of the plains.—Slowness of the elevatory process.—Mode of
formation of the step-formed plains.—Summary.- -Great shingle formation
of Patagonia; its extent, origin, and distribution.—Formation of
sea-cliffs.
CHAPTER II.
ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.
Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions
of the inhabitants on this subject.—Concepcion, earthquake and
elevation of.—VALPARAISO, great elevation of, upraised shells, earth or
marine origin, gradual rise of the land within the historical
period.—COQUIMBO, elevation of, in recent times; terraces of marine
origin, their inclination, their escarpments not horizontal.—Guasco,
gravel terraces of.—Copiapo.—PERU.— Upraised shells of Cobija, Iquique,
and Arica.—Lima, shell-beds and sea- beach on San Lorenzo.—Human
remains, fossil earthenware, earthquake debacle, recent subsidence.—On
the decay of upraised shells.—General summary.
CHAPTER III.
ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL DEPOSITS.
Basin-like plains of Chile; their drainage, their marine origin.—Marks
of sea-action on the eastern flanks of the Cordillera.—Sloping
terrace-like fringes of stratified shingle within the valleys of the
Cordillera; their marine origin.—Boulders in the valley of
Cachapual.—Horizontal elevation of the Cordillera.—Formation of
valleys.—Boulders moved by earthquake- waves.—Saline superficial
deposits.—Bed of nitrate of soda at Iquique.— Saline
incrustations.—Salt-lakes of La Plata and Patagonia; purity of the
salt; its origin.
CHAPTER IV.
ON THE FORMATIONS OF THE PAMPAS.
Mineralogical constitution.—Microscopical structure.—Buenos Ayres,
shells embedded in tosca-rock.—Buenos Ayres to the Colorado.—S.
Ventana.—Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta,
shells, bones, and infusoria of; co-existence of the recent shells and
extinct mammifers.— Buenos Ayres to St. Fe.—Skeletons of
Mastodon.—Infusoria.—Inferior marine tertiary strata, their
age.—Horse’s tooth. BANDA ORIENTAL.— Superficial Pampean
formation.—Inferior tertiary strata, variation of, connected with
volcanic action; Macrauchenia Patachonica at S. Julian in Patagonia,
age of, subsequent to living mollusca and to the erratic block period.
SUMMARY.—Area of Pampean formation.—Theories of origin.—Source of
sediment.—Estuary origin.—Contemporaneous with existing mollusca.—
Relations to underlying tertiary strata. Ancient deposit of estuary
origin.—Elevation and successive deposition of the Pampean formation.—
Number and state of the remains of mammifers; their habitation, food,
extinction, and range.—Conclusion.—Supplement on the thickness of the
Pampean formation.—Localities in Pampas at which mammiferous remains
have been found.
CHAPTER V.
ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.
Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with
infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P. Gallegos.—
Eastern Tierra del Fuego; leaves of extinct beech-trees.—Summary on the
Patagonian tertiary formations.—Tertiary formations of the Western
Coast.—Chonos and Chiloe groups, volcanic rocks
of.—Concepcion.—Navidad.— Coquimbo.—Summary.—Age of the tertiary
formations.—Lines of elevation.— Silicified wood.—Comparative ranges of
the extinct and living mollusca on the West Coast of S.
America.—Climate of the tertiary period.—On the causes of the absence
of recent conchiferous deposits on the coasts of South America.—On the
contemporaneous deposition and preservation of sedimentary formations.
CHAPTER VI.
PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.
Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of
foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in,
decomposition of.—La Plata, metamorphic and old volcanic rocks of.—S.
Ventana.—Claystone porphyry formation of Patagonia; singular
metamorphic rocks; pseudo-dikes.—Falkland Islands, palaeozoic fossils
of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of;
cleavage and foliation; form of land.—Chonos Archipelago, mica-schists,
foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, dikes,
successive formation of.—Central and Northern Chile.—Concluding remarks
on cleavage and foliation.—Their close analogy and similar
origin.—Stratification of metamorphic schists.—Foliation of intrusive
rocks.—Relation of cleavage and foliation to the lines of tension
during metamorphosis.
CHAPTER VII.
CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.
Central Chile.—Basal formations of the Cordillera.—Origin of the
porphyritic clay-stone conglomerate.—Andesite.—Volcanic rocks.—Section
of the Cordillera by the Peuquenes or Portillo Pass.—Great gypseous
formation.—Peuquenes line; thickness of strata, fossils of.—Portillo
line.—Conglomerate, orthitic granite, mica-schist, volcanic rocks of.—
Concluding remarks on the denudation and elevation of the Portillo
line.— Section by the Cumbre, or Uspallata Pass.—Porphyries.—Gypseous
strata.— Section near the Puente del Inca; fossils of.—Great
subsidence.—Intrusive porphyries.—Plain of Uspallata.—Section of the
Uspallata chain.— Structure and nature of the strata.—Silicified
vertical trees.—Great subsidence.—Granitic rocks of axis.—Concluding
remarks on the Uspallata range; origin subsequent to that of the main
Cordillera; two periods of subsidence; comparison with the Portillo
chain.
CHAPTER VIII.
NORTHERN CHILE.—CONCLUSION.
A Section from Illapel to Combarbala; gypseous formation with
silicified wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up
valley; fossils.—Guasco, fossils of.—Copiapo, section up valley; Las
Amolanas, silicified wood.—Conglomerates, nature of former land,
fossils, thickness of strata, great subsidence.—Valley of Despoblado,
fossils, tufaceous deposit, complicated dislocations of.—Relations
between ancient orifices of eruption and subsequent axes of
injection.—Iquique, Peru, fossils of, salt-deposits.—Metalliferous
veins.—Summary on the porphyritic conglomerate and gypseous
formations.—Great subsidence with partial elevations during the
cretaceo-oolitic period.—On the elevation and structure of the
Cordillera.—Recapitulation on the tertiary series.— Relation between
movements of subsidence and volcanic action.—Pampean formation.—Recent
elevatory movements.—Long-continued volcanic action in the
Cordillera.—Conclusion.
GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA
CRITICAL INTRODUCTION.
Of the remarkable “trilogy” constituted by Darwin’s writings which deal
with the geology of the “Beagle,” the member which has perhaps
attracted least attention, up to the present time is that which treats
of the geology of South America. The actual writing of this book
appears to have occupied Darwin a shorter period than either of the
other volumes of the series; his diary records that the work was
accomplished within ten months, namely, between July 1844 and April
1845; but the book was not actually issued till late in the year
following, the preface bearing the date “September 1846.” Altogether,
as Darwin informs us in his “Autobiography,” the geological books
“consumed four and a half years’ steady work,” most of the remainder of
the ten years that elapsed between the return of the “Beagle,” and the
completion of his geological books being, it is sad to relate, “lost
through illness!”
Concerning the “Geological Observations on South America,” Darwin wrote
to his friend Lyell, as follows:—“My volume will be about 240 pages,
dreadfully dull, yet much condensed. I think whenever you have time to
look through it, you will think the collection of facts on the
elevation of the land and on the formation of terraces pretty good.”
“Much condensed” is the verdict that everyone must endorse, on rising
from the perusal of this remarkable book; but by no means “dull.” The
three and a half years from April 1832 to September 1835, were spent by
Darwin in South America, and were devoted to continuous scientific
work; the problems he dealt with were either purely geological or those
which constitute the borderland between the geological and biological
sciences. It is impossible to read the journal which he kept during
this time without being impressed by the conviction that it contains
all the germs of thought which afterwards developed into the “Origin of
Species.” But it is equally evident that after his return to England,
biological speculations gradually began to exercise a more exclusive
sway over Darwin’s mind, and tended to dispossess geology, which during
the actual period of the voyage certainly engrossed most of his time
and attention. The wonderful series of observations made during those
three and a half years in South America could scarcely be done justice
to, in the 240 pages devoted to their exposition. That he executed the
work of preparing the book on South America in somewhat the manner of a
task, is shown by many references in his letters. Writing to Sir Joseph
Hooker in 1845, he says, “I hope this next summer to finish my South
American Geology, then to get out a little Zoology, and HURRAH FOR MY
SPECIES WORK!”
It would seem that the feeling of disappointment, which Darwin so often
experienced in comparing a book when completed, with the observations
and speculations which had inspired it, was more keenly felt in the
case of his volume on South America than any other. To one friend he
writes, “I have of late been slaving extra hard, to the great
discomfiture of wretched digestive organs, at South America, and thank
all the fates, I have done three-fourths of it. Writing plain English
grows with me more and more difficult, and never attainable. As for
your pretending that you will read anything so dull as my pure
geological descriptions, lay not such a flattering unction on my soul,
for it is incredible.” To another friend he writes, “You do not know
what you threaten when you propose to read it—it is purely geological.
I said to my brother, ‘You will of course read it,’ and his answer was,
‘Upon my life, I would sooner even buy it.’”
In spite of these disparaging remarks, however, we are strongly
inclined to believe that this book, despised by its author, and
neglected by his contemporaries, will in the end be admitted to be one
of Darwin’s chief titles to fame. It is, perhaps, an unfortunate
circumstance that the great success which he attained in biology by the
publication of the “Origin of Species” has, to some extent,
overshadowed the fact that Darwin’s claims as a geologist, are of the
very highest order. It is not too much to say that, had Darwin not been
a geologist, the “Origin of Species” could never have been written by
him. But apart from those geological questions, which have an important
bearing on biological thought and speculation, such as the proofs of
imperfection in the geological record, the relations of the later
tertiary faunas to the recent ones in the same areas, and the apparent
intermingling of types belonging to distant geological epochs, when we
study the palaeontology of remote districts,—there are other purely
geological problems, upon which the contributions made by Darwin are of
the very highest value. I believe that the verdict of the historians of
science will be that if Darwin had not taken a foremost place among the
biologists of this century, his position as a geologist would have been
an almost equally commanding one.
But in the case of Darwin’s principal geological work—that relating to
the origin of the crystalline schists,—geologists were not at the time
prepared to receive his revolutionary teachings. The influence of
powerful authority was long exercised, indeed, to stifle his teaching,
and only now, when this unfortunate opposition has disappeared, is the
true nature and importance of Darwin’s purely geological work beginning
to be recognised.
The two first chapters of the “Geological Observations on South
America,” deal with the proofs which exist of great, but frequently
interrupted, movements of elevation during very recent geological
times. In connection with this subject, Darwin’s particular attention
was directed to the relations between the great earthquakes of South
America—of some of which he had impressive experience—and the permanent
changes of elevation which were taking place. He was much struck by the
rapidity with which the evidence of such great earth movements is
frequently obliterated; and especially with the remarkable way in which
the action of rain-water, percolating through deposits on the earth’s
surface, removes all traces of shells and other calcareous organisms.
It was these considerations which were the parents of the
generalisation that a palaeontological record can only be preserved
during those periods in which long-continued slow subsidence is going
on. This in turn, led to the still wider and more suggestive conclusion
that the geological record as a whole is, and never can be more than, a
series of more or less isolated fragments. The recognition of this
important fact constitutes the keystone to any theory of evolution
which seeks to find a basis in the actual study of the types of life
that have formerly inhabited our globe.
In his third chapter, Darwin gives a number of interesting facts,
collected during his visits to the plains and valleys of Chili, which
bear on the question of the origin of saliferous deposits—the
accumulation of salt, gypsum, and nitrate of soda. This is a problem
that has excited much discussion among geologists, and which, in spite
of many valuable observations, still remains to a great extent very
obscure. Among the important considerations insisted upon by Darwin is
that relating to the absence of marine shells in beds associated with
such deposits. He justly argues that if the strata were formed in
shallow waters, and then exposed by upheaval to subaerial action, all
shells and other calcareous organisms would be removed by solution.
Following Lyell’s method, Darwin proceeds from the study of deposits
now being accumulated on the earth’s surface, to those which have been
formed during the more recent periods of the geological history.
His account of the great Pampean formation, with its wonderful
mammalian remains—Mastodon, Toxodon, Scelidotherium, Macrauchenia,
Megatherium, Megalonyx, Mylodon, and Glyptodon—this full of interest.
His discovery of the remains of a true Equus afforded a remarkable
confirmation of the fact- -already made out in North America—that
species of horse had existed and become extinct in the New World,
before their introduction by the Spaniards in the sixteenth century.
Fully perceiving the importance of the microscope in studying the
nature and origin of such deposits as those of the Pampas, Darwin
submitted many of his specimens both to Dr. Carpenter in this country,
and to Professor Ehrenberg in Berlin. Many very important notes on the
microscopic organisms contained in the formation will be found
scattered through the chapter.
Darwin’s study of the older tertiary formations, with their abundant
shells, and their relics of vegetable life buried under great sheets of
basalt, led him to consider carefully the question of climate during
these earlier periods. In opposition to prevalent views on this
subject, Darwin points out that his observations are opposed to the
conclusion that a higher temperature prevailed universally over the
globe during early geological periods. He argues that “the causes which
gave to the older tertiary productions of the quite temperate zones of
Europe a tropical character, WERE OF A LOCAL CHARACTER AND DID NOT
AFFECT THE WHOLE GLOBE.” In this, as in many similar instances, we see
the beneficial influence of extensive travel in freeing Darwin’s mind
from prevailing prejudices. It was this widening of experience which
rendered him so especially qualified to deal with the great problem of
the origin of species, and in doing so to emancipate himself from ideas
which were received with unquestioning faith by geologists whose
studies had been circumscribed within the limits of Western Europe.
In the Cordilleras of Northern and Central Chili, Darwin, when studying
still older formations, clearly recognised that they contain an
admixture of the forms of life, which in Europe are distinctive of the
Cretaceous and Jurassic periods respectively. He was thus led to
conclude that the classification of geological periods, which fairly
well expresses the facts that had been discovered in the areas where
the science was first studied, is no longer capable of being applied
when we come to the study of widely distant regions. This important
conclusion led up to the further generalisation that each great
geological period has exhibited a geographical distribution of the
forms of animal and vegetable life, comparable to that which prevails
in the existing fauna and flora. To those who are familiar with the
extent to which the doctrine of universal formations has affected
geological thought and speculation, both long before and since the time
that Darwin wrote, the importance of this new standpoint to which he
was able to attain will be sufficiently apparent. Like the idea of the
extreme imperfection of the Geological Record, the doctrine of LOCAL
geological formations is found permeating and moulding all the
palaeontological reasonings of his great work.
In one of Darwin’s letters, written while he was in South America,
there is a passage we have already quoted, in which he expresses his
inability to decide between the rival claims upon his attention of “the
old crystalline group of rocks,” and “the softer fossiliferous beds”
respectively. The sixth chapter of the work before us, entitled
“Plutonic and Metamorphic Rocks—Cleavage and Foliation,” contains a
brief summary of a series of observations and reasonings upon these
crystalline rocks, which are, we believe, calculated to effect a
revolution in geological science, and— though their value and
importance have long been overlooked—are likely to entitle Darwin in
the future to a position among geologists, scarcely, if at all,
inferior to that which he already occupies among biologists.
Darwin’s studies of the great rock-masses of the Andes convinced him of
the close relations between the granitic or Plutonic rocks, and those
which were undoubtedly poured forth as lavas. Upon his return, he set
to work, with the aid of Professor Miller, to make a careful study of
the minerals composing the granites and those which occur in the lavas,
and he was able to show that in all essential respects they are
identical. He was further able to prove that there is a complete
gradation between the highly crystalline or granitic rock-masses, and
those containing more or less glassy matter between their crystals,
which constitute ordinary lavas. The importance of this conclusion will
be realised when we remember that it was then the common creed of
geologists—and still continues to be so on the Continent—that all
highly crystalline rocks are of great geological antiquity, and that
the igneous ejections which have taken place since the beginning of the
tertiary periods differ essentially, in their composition, their
structure, and their mode of occurrence, from those which have made
their appearance at earlier periods of the world’s history.
Very completely have the conclusions of Darwin upon these subjects been
justified by recent researches. In England, the United States, and
Italy, examples of the gradual passage of rocks of truly granitic
structure into ordinary lavas have been described, and the reality of
the transition has been demonstrated by the most careful studies with
the microscope. Recent researches carried on in South America by
Professor Stelzner, have also shown the existence of a class of highly
crystalline rocks—the “Andengranites”—which combine in themselves many
of the characteristics which were once thought to be distinctive of the
so-called Plutonic and volcanic rocks. No one familiar with recent
geological literature—even in Germany and France, where the old views
concerning the distinction of igneous products of different ages have
been most stoutly maintained—can fail to recognise the fact that the
principles contended for by Darwin bid fair at no distant period to win
universal acceptance among geologists all over the globe.
Still more important are the conclusions at which Darwin arrived with
respect to the origin of the schists and gneisses which cover so large
an area in South America.
Carefully noting, by the aid of his compass and clinometer, at every
point which he visited, the direction and amount of inclination of the
parallel divisions in these rocks, he was led to a very important
generalisation— namely, that over very wide areas the direction
(strike) of the planes of cleavage in slates, and of foliation in
schists and gneisses, remained constant, though the amount of their
inclination (dip) often varied within wide limits. Further than this it
appeared that there was always a close correspondence between the
strike of the cleavage and foliation and the direction of the great
axes along which elevation had taken place in the district.
In Tierra del Fuego, Darwin found striking evidence that the cleavage
intersecting great masses of slate-rocks was quite independent of their
original stratification, and could often, indeed, be seen cutting
across it at right angles. He was also able to verify Sedgwick’s
observation that, in some slates, glossy surfaces on the planes of
cleavage arise from the development of new minerals, chlorite, epidote
or mica, and that in this way a complete graduation from slates to true
schists may be traced.
Darwin further showed that in highly schistose rocks, the folia bend
around and encircle any foreign bodies in the mass, and that in some
cases they exhibit the most tortuous forms and complicated puckerings.
He clearly saw that in all cases the forces by which these striking
phenomena must have been produced were persistent over wide areas, and
were connected with the great movements by which the rocks had been
upheaved and folded.
That the distinct folia of quartz, feldspar, mica, and other minerals
composing the metamorphic schists could not have been separately
deposited as sediment was strongly insisted upon by Darwin; and in
doing so he opposed the view generally prevalent among geologists at
that time. He was thus driven to the conclusion that foliation, like
cleavage, is not an original, but a superinduced structure in
rock-masses, and that it is the result of re-crystallisation, under the
controlling influence of great pressure, of the materials of which the
rock was composed.
In studying the lavas of Ascension, as we have already seen, Darwin was
led to recognise the circumstance that, when igneous rocks are
subjected to great differential movements during the period of their
consolidation, they acquire a foliated structure, closely analogous to
that of the crystalline schists. Like his predecessor in this field of
inquiry, Mr. Poulett Scrope, Charles Darwin seems to have been greatly
impressed by these facts, and he argued from them that the rocks
exhibiting the foliated structure must have been in a state of
plasticity, like that of a cooling mass of lava. At that time the
suggestive experiments of Tresca, Daubree, and others, showing that
solid masses under the influence of enormous pressure become actually
plastic, had not been published. Had Darwin been aware of these facts
he would have seen that it was not necessary to assume a state of
imperfect solidity in rock-masses in order to account for their having
yielded to pressure and tension, and, in doing so, acquiring the new
characters which distinguish the crystalline schists.
The views put forward by Darwin on the origin of the crystalline
schists found an able advocate in Mr. Daniel Sharpe, who in 1852 and
1854 published two papers, dealing with the geology of the Scottish
Highlands and of the Alps respectively, in which he showed that the
principles arrived at by Darwin when studying the South American rocks
afford a complete explanation of the structure of the two districts in
question.
But, on the other hand, the conclusions of Darwin and Sharpe were met
with the strongest opposition by Sir Roderick Murchison and Dr. A.
Geikie, who in 1861 read a paper before the Geological Society “On the
Coincidence between Stratification and Foliation in the Crystalline
Rocks of the Scottish Highlands,” in which they insisted that their
observations in Scotland tended to entirely disprove the conclusions of
Darwin that foliation in rocks is a secondary structure, and entirely
independent of the original stratification of the rock-masses.
Now it is a most significant circumstance that, no sooner did the
officers of the Geological Survey commence the careful and detailed
study of the Scottish Highlands than they found themselves compelled to
make a formal retraction of the views which had been put forward by
Murchison and Geikie in opposition to the conclusions of Darwin. The
officers of the Geological Survey have completely abandoned the view
that the foliation of the Highland rocks has been determined by their
original stratification, and admit that the structure is the result of
the profound movements to which the rocks have been subjected. The same
conclusions have recently been supported by observations made in many
different districts—among which we may especially refer to those of Dr.
H. Reusch in Norway, and those of Dr. J. Lehmann in Saxony. At the
present time the arguments so clearly stated by Darwin in the work
before us, have, after enduring opposition or neglect for a whole
generation, begun to “triumph all along the line,” and we may look
forward confidently to the near future, when his claim to be regarded
as one of the greatest of geological discoverers shall be fully
vindicated.
JOHN W. JUDD.
CHAPTER I.
ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.
Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and
Pumice-pebbles.—Step-formed plains of Patagonia, with upraised
Shells.—Terrace-bounded Valley of Santa Cruz, formerly a
Sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth of
the elevated area.—Equability of the movements, as shown by the similar
heights of the plains.—Slowness of the elevatory process.—Mode of
formation of the step-formed plains.—Summary.—Great Shingle Formation
of Patagonia; its extent, origin, and distribution.—Formation of
sea-cliffs.
In the following Volume, which treats of the geology of South America,
and almost exclusively of the parts southward of the Tropic of
Capricorn, I have arranged the chapters according to the age of the
deposits, occasionally departing from this order, for the sake of
geographical simplicity.
The elevation of the land within the recent period, and the
modifications of its surface through the action of the sea (to which
subjects I paid particular attention) will be first discussed; I will
then pass on to the tertiary deposits, and afterwards to the older
rocks. Only those districts and sections will be described in detail
which appear to me to deserve some particular attention; and I will, at
the end of each chapter, give a summary of the results. We will
commence with the proofs of the upheaval of the eastern coast of the
continent, from the Rio Plata southward; and, in the Second Chapter,
follow up the same subject along the shores of Chile and Peru.
On the northern bank of the great estuary of the Rio Plata, near
Maldonado, I found at the head of a lake, sometimes brackish but
generally containing fresh water, a bed of muddy clay, six feet in
thickness, with numerous shells of species still existing in the Plata,
namely, the Azara labiata, d’Orbigny, fragments of Mytilus eduliformis,
d’Orbigny, Paludestrina Isabellei, d’Orbigny, and the Solen Caribaeus,
Lam., which last was embedded vertically in the position in which it
had lived. These shells lie at the height of only two feet above the
lake, nor would they have been worth mentioning, except in connection
with analogous facts.
At Monte Video, I noticed near the town, and along the base of the
mount, beds of a living Mytilus, raised some feet above the surface of
the Plata: in a similar bed, at a height from thirteen to sixteen feet,
M. Isabelle collected eight species, which, according to M. d’Orbigny,
now live at the mouth of the estuary. (“Voyage dans l’Amerique Merid.:
Part. Geolog.” page 21.) At Colonia del Sacramiento, further westward,
I observed at the height of about fifteen feet above the river, there
of quite fresh water, a small bed of the same Mytilus, which lives in
brackish water at Monte Video. Near the mouth of Uruguay, and for at
least thirty-five miles northward, there are at intervals large sandy
tracts, extending several miles from the banks of the river, but not
raised much above its level, abounding with small bivalves, which occur
in such numbers that at the Agraciado they are sifted and burnt for
lime. Those which I examined near the A. S. Juan were much worn: they
consisted of Mactra Isabellei, d’Orbigny, mingled with few of Venus
sinuosa, Lam., both inhabiting, as I am informed by M. d’Orbigny,
brackish water at the mouth of the Plata, nearly or quite as salt as
the open sea. The loose sand, in which these shells are packed, is
heaped into low, straight, long lines of dunes, like those left by the
sea at the head of many bays. M. d’Orbigny has described an analogous
phenomenon on a greater scale, near San Pedro on the river Parana,
where he found widely extended beds and hillocks of sand, with vast
numbers of the Azara labiata, at the height of nearly 100 feet
(English) above the surface of that river. (Ibid page 43.) The Azara
inhabits brackish water, and is not known to be found nearer to San
Pedro than Buenos Ayres, distant above a hundred miles in a straight
line. Nearer Buenos Ayres, on the road from that place to San Isidro,
there are extensive beds, as I am informed by Sir Woodbine Parish, of
the Azara labiata, lying at about forty feet above the level of the
river, and distant between two and three miles from it. (“Buenos Ayres”
etc. by Sir Woodbine Parish page 168.) These shells are always found on
the highest banks in the district: they are embedded in a stratified
earthy mass, precisely like that of the great Pampean deposit hereafter
to be described. In one collection of these shells, there were some
valves of the Venus sinuosa, Lam., the same species found with the
Mactra on the banks of the Uruguay. South of Buenos Ayres, near
Ensenada, there are other beds of the Azara, some of which seem to have
been embedded in yellowish, calcareous, semi-crystalline matter; and
Sir W. Parish has given me from the banks of the Arroyo del Tristan,
situated in this same neighbourhood, at the distance of about a league
from the Plata, a specimen of a pale- reddish, calcereo-argillaceous
stone (precisely like parts of the Pampean deposit the importance of
which fact will be referred to in a succeeding chapter), abounding with
shells of an Azara, much worn, but which in general form and appearance
closely resemble, and are probably identical with, the A. labiata.
Besides these shells, cellular, highly crystalline rock, formed of the
casts of small bivalves, is found near Ensenada; and likewise beds of
sea-shells, which from their appearance appear to have lain on the
surface. Sir W. Parish has given me some of these shells, and M.
d’Orbigny pronounces them to be:—
1. Buccinanops globulosum, d’Orbigny.
2. Olivancillaria auricularia, d’Orbigny.
3. Venus flexuosa, Lam.
4. Cytheraea (imperfect).
5. Mactra Isabellei, d’Orbigny.
6. Ostrea pulchella, d’Orbigny.
Besides these, Sir W. Parish procured (“Buenos Ayres” etc. by Sir W.
Parish page 168.) (as named by Mr. G.B. Sowerby) the following shells:—
7. Voluta colocynthis.
8. Voluta angulata.
9. Buccinum (not spec.?).
All these species (with, perhaps, the exception of the last) are
recent, and live on the South American coast. These shell-beds extend
from one league to six leagues from the Plata, and must lie many feet
above its level. I heard, also, of beds of shells on the Somborombon,
and on the Rio Salado, at which latter place, as M. d’Orbigny informs
me, the Mactra Isabellei and Venus sinuosa are found.
During the elevation of the Provinces of La Plata, the waters of the
ancient estuary have but little affected (with the exception of the
sand- hills on the banks of the Parana and Uruguay) the outline of the
land. M. Parchappe, however, has described groups of sand dunes
scattered over the wide extent of the Pampas southward of Buenos Ayres
(D’Orbigny “Voyage Geolog.” page 44.), which M. d’Orbigny attributes
with much probability to the action of the sea, before the plains were
raised above its level. (Before proceeding to the districts southward
of La Plata, it may be worth while just to state, that there is some
evidence that the coast of Brazil has participated in a small amount of
elevation. Mr. Burchell informs me, that he collected at Santos
(latitude 24 degrees S.) oyster-shells, apparently recent, some miles
from the shore, and quite above the tidal action. Westward of Rio de
Janeiro, Captain Elliot is asserted (see Harlan “Med. and Phys. Res.”
page 35 and Dr. Meigs in “Transactions of the American Philosophical
Society”), to have found human bones, encrusted with sea-shells,
between fifteen and twenty feet above the level of the sea. Between Rio
de Janeiro and Cape Frio I crossed sandy tracts abounding with
sea-shells, at a distance of a league from the coast; but whether these
tracts have been formed by upheaval, or through the mere accumulation
of drift sand, I am not prepared to assert. At Bahia (latitude 13
degrees S.), in some parts near the coast, there are traces of
sea-action at the height of about twenty feet above its present level;
there are also, in many parts, remnants of beds of sandstone and
conglomerate with numerous recent shells, raised a little above the
sea-level. I may add, that at the head of Bahia Bay there is a
formation, about forty feet in thickness, containing tertiary shells
apparently of fresh-water origin, now washed by the sea and encrusted
with Balini; this appears to indicate a small amount of subsidence
subsequent to its deposition. At Pernambuco (latitude 8 degrees S.), in
the alluvial or tertiary cliffs, surrounding the low land on which the
city stands, I looked in vain for organic remains, or other evidence of
changes in level.)
SOUTHWARD OF THE PLATA.
The coast as far as Bahia Blanca (in latitude 39 degrees S.) is formed
either of a horizontal range of cliffs, or of immense accumulations of
sand-dunes. Within Bahia Blanca, a small piece of tableland, about
twenty feet above high-water mark, called Punta Alta, is formed of
strata of cemented gravel and of red earthy mud, abounding with shells
(with others lying loose on the surface), and the bones of extinct
mammifers. These shells, twenty in number, together with a Balanus and
two corals, are all recent species, still inhabiting the neighbouring
seas. They will be enumerated in the Fourth Chapter, when describing
the Pampean formation; five of them are identical with the upraised
ones from near Buenos Ayres. The northern shore of Bahia Blanca is, in
main part, formed of immense sand-dunes, resting on gravel with recent
shells, and ranging in lines parallel to the shore. These ranges are
separated from each other by flat spaces, composed of stiff impure red
clay, in which, at the distance of about two miles from the coast, I
found by digging a few minute fragments of sea-shells. The sand-dunes
extend several miles inland, and stand on a plain, which slopes up to a
height of between one hundred and two hundred feet. Numerous, small,
well-rounded pebbles of pumice lie scattered both on the plain and
sand-hillocks: at Monte Hermoso, on the flat summit of a cliff, I found
many of them at a height of 120 feet (angular measurement) above the
level of the sea. These pumice pebbles, no doubt, were originally
brought down from the Cordillera by the rivers which cross the
continent, in the same way as the river Negro anciently brought down,
and still brings down, pumice, and as the river Chupat brings down
scoriae: when once delivered at the mouth of a river, they would
naturally have travelled along the coasts, and been cast up during the
elevation of the land, at different heights. The origin of the
argillaceous flats, which separate the parallel ranges of sand-dunes,
seems due to the tides here having a tendency (as I believe they have
on most shoal, protected coasts) to throw up a bar parallel to the
shore, and at some distance from it; this bar gradually becomes larger,
affording a base for the accumulation of sand- dunes, and the shallow
space within then becomes silted up with mud. The repetition of this
process, without any elevation of the land, would form a level plain
traversed by parallel lines of sand-hillocks; during a slow elevation
of the land, the hillocks would rest on a gently inclined surface, like
that on the northern shore of Bahia Blanca. I did not observe any
shells in this neighbourhood at a greater height than twenty feet; and
therefore the age of the sea-drifted pebbles of pumice, now standing at
the height of 120 feet, must remain uncertain.
The main plain surrounding Bahia Blanca I estimated at from two hundred
to three hundred feet; it insensibly rises towards the distant Sierra
Ventana. There are in this neighbourhood some other and lower plains,
but they do not abut one at the foot of the other, in the manner
hereafter to be described, so characteristic of Patagonia. The plain on
which the settlement stands is crossed by many low sand-dunes,
abounding with the minute shells of the Paludestrina australis,
d’Orbigny, which now lives in the bay. This low plain is bounded to the
south, at the Cabeza del Buey, by the cliff-formed margin of a wide
plain of the Pampean formation, which I estimated at sixty feet in
height. On the summit of this cliff there is a range of high sand-dunes
extending several miles in an east and west line.
Southward of Bahia Blanca, the river Colorado flows between two plains,
apparently from thirty to forty feet in height. Of these plains, the
southern one slopes up to the foot of the great sandstone plateau of
the Rio Negro; and the northern one against an escarpment of the
Pampean deposit; so that the Colorado flows in a valley fifty miles in
width, between the upper escarpments. I state this, because on the low
plain at the foot of the northern escarpment, I crossed an immense
accumulation of high sand-dunes, estimated by the Gauchos at no less
than eight miles in breadth. These dunes range westward from the coast,
which is twenty miles distant, to far inland, in lines parallel to the
valley; they are separated from each other by argillaceous flats,
precisely like those on the northern shore of Bahia Blanca. At present
there is no source whence this immense accumulation of sand could
proceed; but if, as I believe, the upper escarpments once formed the
shores of an estuary, in that case the sandstone formation of the river
Negro would have afforded an inexhaustible supply of sand, which would
naturally have accumulated on the northern shore, as on every part of
the coast open to the south winds between Bahia Blanca and Buenos
Ayres.
At San Blas (40 degrees 40′ S.) a little south of the mouth of the
Colorado, M. d’Orbigny found fourteen species of existing shells (six
of them identical with those from Bahia Blanca), embedded in their
natural positions. (“Voyage” etc. page 54.) From the zone of depth
which these shells are known to inhabit, they must have been uplifted
thirty-two feet. He also found, at from fifteen to twenty feet above
this bed, the remains of an ancient beach.
Ten miles southward, but 120 miles to the west, at Port S. Antonio, the
Officers employed on the Survey assured me that they saw many old sea-
shells strewed on the surface of the ground, similar to those found on
other parts of the coast of Patagonia. At San Josef, ninety miles south
in nearly the same longitude, I found, above the gravel, which caps an
old tertiary formation, an irregular bed and hillock of sand, several
feet in thickness, abounding with shells of Patella deaurita, Mytilus
Magellanicus, the latter retaining much of its colour; Fusus
Magellanicus (and a variety of the same), and a large Balanus (probably
B. Tulipa), all now found on this coast: I estimated this bed at from
eighty to one hundred feet above the level of the sea. To the westward
of this bay, there is a plain estimated at between two hundred and
three hundred feet in height: this plain seems, from many measurements,
to be a continuation of the sandstone platform of the river Negro. The
next place southward, where I landed, was at Port Desire, 340 miles
distant; but from the intermediate districts I received, through the
kindness of the Officers of the Survey, especially from Lieutenant
Stokes and Mr. King, many specimens and sketches, quite sufficient to
show the general uniformity of the whole line of coast. I may here
state, that the whole of Patagonia consists of a tertiary formation,
resting on and sometimes surrounding hills of porphyry and quartz: the
surface is worn into many wide valleys and into level step-formed
plains, rising one above another, all capped by irregular beds of
gravel, chiefly composed of porphyritic rocks. This gravel formation
will be separately described at the end of the chapter.
My object in giving the following measurements of the plains, as taken
by the Officers of the Survey, is, as will hereafter be seen, to show
the remarkable equability of the recent elevatory movements. Round the
southern parts of Nuevo Gulf, as far as the River Chupat (seventy miles
southward of San Josef), there appear to be several plains, of which
the best defined are here represented.
(In the following Diagrams: 1. Baseline is Level of sea. 2. Scale is
1/20 of inch to 100 feet vertical. 3. Height is shown in feet thus: An.
M. always stands for angular or trigonometrical measurement. Ba. M.
always stands for barometrical measurement. Est. always stands for
estimation by the Officers of the Survey.
DIAGRAM 1. SECTION OF STEP-FORMED PLAINS SOUTH OF NUEVO GULF.
From East (sea level) to West (high): Terrace 1. 80 Est. Terrace 2.
200-220 An. M. Terrace 3. 350 An. M.)
The upper plain is here well defined (called Table Hills); its edge
forms a cliff or line of escarpment many miles in length, projecting
over a lower plain. The lowest plain corresponds with that at San Josef
with the recent shells on its surface. Between this lowest and the
uppermost plain, there is probably more than one step-formed terrace:
several measurements show the existence of the intermediate one of the
height given in Diagram 1.
(DIAGRAM 2. SECTION OF PLAINS IN THE BAY OF ST. GEORGE.
From East (sea level) to West (high): Terrace 1. 250 An. M. Terrace 2.
330 An. M. Terrace 3. 580 An. M. Terraces 4, 5 and 6 not measured.
Terrace 7. 1,200 Est.)
Near the north headland of the great Bay of St. George (100 miles south
of the Chupat), two well-marked plains of 250 and 330 feet were
measured: these are said to sweep round a great part of the Bay. At its
south headland, 120 miles distant from the north headland, the 250 feet
plain was again measured. In the middle of the bay, a higher plain was
found at two neighbouring places (Tilli Roads and C. Marques) to be 580
feet in height. Above this plain, towards the interior, Mr. Stokes
informs me that there were several other step-formed plains, the
highest of which was estimated at 1,200 feet, and was seen ranging at
apparently the same height for 150 miles northward. All these plains
have been worn into great valleys and much denuded. The section in
Diagram 3 is illustrative of the general structure of the great Bay of
St. George. At the south headland of the Bay of St. George (near C.
Three Points) the 250 plain is very extensive.
(DIAGRAM 3. SECTION OF PLAINS AT PORT DESIRE.
From East (sea level) to West (high): Terrace 1. 100 Est. Terrace 2.
245-255 Ba. M. Shells on surface. Terrace 3. 330 Ba. M. Shells on
surface. Terrace 4. Not measured.)
At Port Desire (forty miles southward) I made several measurements with
the barometer of a plain, which extends along the north side of the
port and along the open coast, and which varies from 245 to 255 feet in
height: this plain abuts against the foot of a higher plain of 330
feet, which extends also far northward along the coast, and likewise
into the interior. In the distance a higher inland platform was seen,
of which I do not know the height. In three separate places, I observed
the cliff of the 245-255 feet plain, fringed by a terrace or narrow
plain estimated at about one hundred feet in height. These plains are
represented in the section Diagram 3.
In many places, even at the distance of three and four miles from the
coast, I found on the gravel-capped surface of the 245-255 feet, and of
the 330 feet plain, shells of Mytilus Magellanicus, M. edulis, Patella
deaurita, and another Patella, too much worn to be identified, but
apparently similar to one found abundantly adhering to the leaves of
the kelp. These species are the commonest now living on this coast. The
shells all appeared very old; the blue of the mussels was much faded;
and only traces of colour could be perceived in the Patellas, of which
the outer surfaces were scaling off. They lay scattered on the smooth
surface of the gravel, but abounded most in certain patches, especially
at the heads of the smaller valleys: they generally contained sand in
their insides; and I presume that they have been washed by alluvial
action out of thin sandy layers, traces of which may sometimes be seen
covering the gravel. The several plains have very level surfaces; but
all are scooped out by numerous broad, winding, flat-bottomed valleys,
in which, judging from the bushes, streams never flow. These remarks on
the state of the shells, and on the nature of the plains, apply to the
following cases, so need not be repeated.
(DIAGRAM 4. SECTION OF PLAINS AT PORT S. JULIAN.
From East (sea level) to West (high): Terrace 1. Shells on surface. 90
Est. Terrace 2. 430 An. M. Terrace 3. 560 An. M. Terrace 4. 950 An. M.)
Southward of Port Desire, the plains have been greatly denuded, with
only small pieces of tableland marking their former extension. But
opposite Bird Island, two considerable step-formed plains were
measured, and found respectively to be 350 and 590 feet in height. This
latter plain extends along the coast close to Port St. Julian (110
miles south of Port Desire); see Diagram 4.
The lowest plain was estimated at ninety feet: it is remarkable from
the usual gravel-bed being deeply worn into hollows, which are filled
up with, as well as the general surface covered by, sandy and reddish
earthy matter: in one of the hollows thus filled up, the skeleton of
the Macrauchenia Patachonica, as will hereafter be described, was
embedded. On the surface and in the upper parts of this earthy mass,
there were numerous shells of Mytilus Magellanicus and M. edulis,
Patella deaurita, and fragments of other species. This plain is
tolerably level, but not extensive; it forms a promontory seven or
eight miles long, and three or four wide. The upper plains in Diagram 4
were measured by the Officers of the Survey; they were all capped by
thick beds of gravel, and were all more or less denuded; the 950 plain
consists merely of separate, truncated, gravel-capped hills, two of
which, by measurement, were found to differ only three feet. The 430
feet plain extends, apparently with hardly a break, to near the
northern entrance of the Rio Santa Cruz (fifty miles to the south); but
it was there found to be only 330 feet in height.
(DIAGRAM 5. SECTION OF PLAINS AT THE MOUTH OF THE RIO SANTA CRUZ.
From East (sea level) to West (high): Terrace 1. (sloping) 355 Ba. M.
Shells on surface. 463 Ba. M. Terrace 2. 710 An. M. Terrace 3. 840 An.
M.)
On the southern side of the mouth of the Santa Cruz we have Diagram 5,
which I am able to give with more detail than in the foregoing cases.
The plain marked 355 feet (as ascertained by the barometer and by
angular measurement) is a continuation of the above-mentioned 330 feet
plain: it extends in a N.W. direction along the southern shores of the
estuary. It is capped by gravel, which in most parts is covered by a
thin bed of sandy earth, and is scooped out by many flat-bottomed
valleys. It appears to the eye quite level, but in proceeding in a
S.S.W. course, towards an escarpment distant about six miles, and
likewise ranging across the country in a N.W. line, it was found to
rise at first insensibly, and then for the last half-mile, sensibly,
close up to the base of the escarpment: at this point it was 463 feet
in height, showing a rise of 108 feet in the six miles. On this 355-463
feet plain, I found several shells of Mytilus Magellanicus and of a
Mytilus, which Mr. Sowerby informs me is yet unnamed, though well-known
as recent on this coast; Patella deaurita; Fusus, I believe,
Magellanicus, but the specimen has been lost; and at the distance of
four miles from the coast, at the height of about four hundred feet,
there were fragments of the same Patella and of a Voluta (apparently V.
ancilla) partially embedded in the superficial sandy earth. All these
shells had the same ancient appearance with those from the foregoing
localities. As the tides along this part of the coast rise at the
Syzygal period forty feet, and therefore form a well-marked beach-line,
I particularly looked out for ridges in crossing this plain, which, as
we have seen, rises 108 feet in about six miles, but I could not see
any traces of such. The next highest plain is 710 feet above the sea;
it is very narrow, but level, and is capped with gravel; it abuts to
the foot of the 840 feet plain. This summit-plain extends as far as the
eye can range, both inland along the southern side of the valley of the
Santa Cruz, and southward along the Atlantic.
THE VALLEY OF THE R. SANTA CRUZ.
This valley runs in an east and west direction to the Cordillera, a
distance of about one hundred and sixty miles. It cuts through the
great Patagonian tertiary formation, including, in the upper half of
the valley, immense streams of basaltic lava, which as well as the
softer beds, are capped by gravel; and this gravel, high up the river,
is associated with a vast boulder formation. (I have described this
formation in a paper in the “Geological Transactions” volume 6 page
415.) In ascending the valley, the plain which at the mouth on the
southern side is 355 feet high, is seen to trend towards the
corresponding plain on the northern side, so that their escarpments
appear like the shores of a former estuary, larger than the existing
one: the escarpments, also, of the 840 feet summit-plain (with a
corresponding northern one, which is met with some way up the valley),
appear like the shores of a still larger estuary. Farther up the
valley, the sides are bounded throughout its entire length by level,
gravel-capped terraces, rising above each other in steps. The width
between the upper escarpments is on an average between seven and ten
miles; in one spot, however, where cutting through the basaltic lava,
it was only one mile and a half. Between the escarpments of the second
highest terrace the average width is about four or five miles. The
bottom of the valley, at the distance of 110 miles from its mouth,
begins sensibly to expand, and soon forms a considerable plain, 440
feet above the level of the sea, through which the river flows in a gut
from twenty to forty feet in depth. I here found, at a point 140 miles
from the Atlantic, and seventy miles from the nearest creek of the
Pacific, at the height of 410 feet, a very old and worn shell of
Patella deaurita. Lower down the valley, 105 miles from the Atlantic
(longitude 71 degrees W.), and at an elevation of about 300 feet, I
also found, in the bed of the river, two much worn and broken shells of
the Voluta ancilla, still retaining traces of their colours; and one of
the Patella deaurita. It appeared that these shells had been washed
from the banks into the river; considering the distance from the sea,
the desert and absolutely unfrequented character of the country, and
the very ancient appearance of the shells (exactly like those found on
the plains nearer the coast), there is, I think, no cause to suspect
that they could have been brought here by Indians.
The plain at the head of the valley is tolerably level, but water-worn,
and with many sand-dunes on it like those on a sea-coast. At the
highest point to which we ascended, it was sixteen miles wide in a
north and south line; and forty-five miles in length in an east and
west line. It is bordered by the escarpments, one above the other, of
two plains, which diverge as they approach the Cordillera, and
consequently resemble, at two levels, the shores of great bays facing
the mountains; and these mountains are breached in front of the lower
plain by a remarkable gap. The valley, therefore, of the Santa Cruz
consists of a straight broad cut, about ninety miles in length,
bordered by gravel-capped terraces and plains, the escarpments of which
at both ends diverge or expand, one over the other, after the manner of
the shores of great bays. Bearing in mind this peculiar form of the
land—the sand-dunes on the plain at the head of the valley—the gap in
the Cordillera, in front of it—the presence in two places of very
ancient shells of existing species—and lastly, the circumstance of the
355-453 feet plain, with the numerous marine remains on its surface,
sweeping from the Atlantic coast, far up the valley, I think we must
admit, that within the recent period, the course of the Santa Cruz
formed a sea-strait intersecting the continent. At this period, the
southern part of South America consisted of an archipelago of islands
360 miles in a north and south line. We shall presently see, that two
other straits also, since closed, then cut through Tierra del Fuego; I
may add, that one of them must at that time have expanded at the foot
of the Cordillera into a great bay (now Otway Water) like that which
formerly covered the 440 feet plain at the head of the Santa Cruz.
(DIAGRAM 6. NORTH AND SOUTH SECTION ACROSS THE TERRACES BOUNDING THE
VALLEY OF THE RIVER SANTA CRUZ, HIGH UP ITS COURSE.
The height of each terrace, above the level of the river (furthest to
nearest to the river) in feet:
A, north and south: 1,122 B, north and south: 869 C, north and south:
639 D, north: not measured. D, north? (suggest south): 185 E: 20 Bed of
River.
Vertical scale 1/20 of inch to 100 feet; but terrace E, being only
twenty feet above the river, has necessarily been raised. The
horizontal distances much contracted; the distance from the edge of A
North to A South being on an average from seven to ten miles.) I have
said that the valley in its whole course is bordered by gravel- capped
plains. The section (Diagram 6), supposed to be drawn in a north and
south line across the valley, can scarcely be considered as more than
illustrative; for during our hurried ascent it was impossible to
measure all the plains at any one place. At a point nearly midway
between the Cordillera and the Atlantic, I found the plain (A north)
1,122 feet above the river; all the lower plains on this side were here
united into one great broken cliff: at a point sixteen miles lower down
the stream, I found by measurement and estimation that B (north) was
869 above the river: very near to where A (north) was measured, C
(north) was 639 above the same level: the terrace D (north) was nowhere
measured: the lowest E (north) was in many places about twenty feet
above the river. These plains or terraces were best developed where the
valley was widest; the whole five, like gigantic steps, occurred
together only at a few points. The lower terraces are less continuous
than the higher ones, and appear to be entirely lost in the upper third
of the valley. Terrace C (south), however was traced continuously for a
great distance. The terrace B (north), at a point fifty- five miles
from the mouth of the river, was four miles in width; higher up the
valley this terrace (or at least the second highest one, for I could
not always trace it continuously) was about eight miles wide. This
second plain was generally wider than the lower ones—as indeed follows
from the valley from A (north) to A (south) being generally nearly
double the width of from B (north) to B (south). Low down the valley,
the summit-plain A (south) is continuous with the 840 feet plain on the
coast, but it is soon lost or unites with the escarpment of B (south).
The corresponding plain A (north), on the north side of the valley,
appears to range continuously from the Cordillera to the head of the
present estuary of the Santa Cruz, where it trends northward towards
Port St. Julian. Near the Cordillera the summit-plain on both sides of
the valley is between 3,200 and 3,300 feet in height; at 100 miles from
the Atlantic, it is 1,416 feet, and on the coast 840 feet, all above
the sea-beach; so that in a distance of 100 miles the plain rises 576
feet, and much more rapidly near to the Cordillera. The lower terraces
B and C also appear to rise as they run up the valley; thus D (north),
measured at two points twenty-four miles apart, was found to have risen
185 feet. From several reasons I suspect, that this gradual inclination
of the plains up the valley, has been chiefly caused by the elevation
of the continent in mass, having been the greater the nearer to the
Cordillera.
All the terraces are capped with well-rounded gravel, which rests
either on the denuded and sometimes furrowed surface of the soft
tertiary deposits, or on the basaltic lava. The difference in height
between some of the lower steps or terraces seems to be entirely owing
to a difference in the thickness of the capping gravel. Furrows and
inequalities in the gravel, where such occur, are filled up and
smoothed over with sandy earth. The pebbles, especially on the higher
plains, are often whitewashed, and even cemented together by a white
aluminous substance, and I occasionally found this to be the case with
the gravel on the terrace D. I could not perceive any trace of a
similar deposition on the pebbles now thrown up by the river, and
therefore I do not think that terrace D was river-formed. As the
terrace E generally stands about twenty feet above the bed of the
river, my first impression was to doubt whether even this lowest one
could have been so formed; but it should always be borne in mind, that
the horizontal upheaval of a district, by increasing the total descent
of the streams, will always tend to increase, first near the sea-coast
and then further and further up the valley, their corroding and
deepening powers: so that an alluvial plain, formed almost on a level
with a stream, will, after an elevation of this kind, in time be cut
through, and left standing at a height never again to be reached by the
water. With respect to the three upper terraces of the Santa Cruz, I
think there can be no doubt, that they were modelled by the sea, when
the valley was occupied by a strait, in the same manner (hereafter to
be discussed) as the greater step-formed, shell- strewed plains along
the coast of Patagonia.
To return to the shores of the Atlantic: the 840 feet plain, at the
mouth of the Santa Cruz, is seen extending horizontally far to the
south; and I am informed by the Officers of the Survey, that bending
round the head of Coy Inlet (sixty-five miles southward), it trends
inland. Outliers of apparently the same height are seen forty miles
farther south, inland of the river Gallegos; and a plain comes down to
Cape Gregory (thirty-five miles southward), in the Strait of Magellan,
which was estimated at between eight hundred and one thousand feet in
height, and which, rising towards the interior, is capped by the
boulder formation. South of the Strait of Magellan, there are large
outlying masses of apparently the same great tableland, extending at
intervals along the eastern coast of Tierra del Fuego: at two places
here, 110 miles a part, this plain was found to be 950 and 970 feet in
height.
From Coy Inlet, where the high summit-plain trends inland, a plain
estimated at 350 feet in height, extends for forty miles to the river
Gallegos. From this point to the Strait of Magellan, and on each side
of that Strait, the country has been much denuded and is less level. It
consists chiefly of the boulder formation, which rises to a height of
between one hundred and fifty and two hundred and fifty feet, and is
often capped by beds of gravel. At N.S. Gracia, on the north side of
the Inner Narrows of the Strait of Magellan, I found on the summit of a
cliff, 160 feet in height, shells of existing Patellae and Mytili,
scattered on the surface and partially embedded in earth. On the
eastern coast, also, of Tierra del Fuego, in latitude 53 degrees 20′
south, I found many Mytili on some level land, estimated at 200 feet in
height. Anterior to the elevation attested by these shells, it is
evident by the present form of the land, and by the distribution of the
great erratic boulders on the surface, that two sea-channels connected
the Strait of Magellan both with Sebastian Bay and with Otway Water.
(“Geological Transactions” volume 6 page 419.)
CONCLUDING REMARKS ON THE RECENT ELEVATION OF THE SOUTH-EASTERN COASTS
OF AMERICA, AND ON THE ACTION OF THE SEA ON THE LAND.
Upraised shells of species, still existing as the commonest kinds in
the adjoining sea, occur, as we have seen, at heights of between a few
feet and 410 feet, at intervals from latitude 33 degrees 40′ to 53
degrees 20′ south. This is a distance of 1,180 geographical miles—about
equal from London to the North Cape of Sweden. As the boulder formation
extends with nearly the same height 150 miles south of 53 degrees 20′,
the most southern point where I landed and found upraised shells; and
as the level Pampas ranges many hundred miles northward of the point,
where M. d’Orbigny found at the height of 100 feet beds of the Azara,
the space in a north and south line, which has been uplifted within the
recent period, must have been much above the 1,180 miles. By the term
“recent,” I refer only to that period within which the now living
mollusca were called into existence; for it will be seen in the Fourth
Chapter, that both at Bahia Blanca and P. S. Julian, the mammiferous
quadrupeds which co-existed with these shells belong to extinct
species. I have said that the upraised shells were found only at
intervals on this line of coast, but this in all probability may be
attributed to my not having landed at the intermediate points; for
wherever I did land, with the exception of the river Negro, shells were
found: moreover, the shells are strewed on plains or terraces, which,
as we shall immediately see, extend for great distances with a uniform
height. I ascended the higher plains only in a few places, owing to the
distance at which their escarpments generally range from the coast, so
that I am far from knowing that 410 feet is the maximum of elevation of
these upraised remains. The shells are those now most abundant in a
living state in the adjoining sea. (Captain King “Voyages of
‘Adventure’ and ‘Beagle’” volume 1 pages 6 and 133.) All of them have
an ancient appearance; but some, especially the mussels, although lying
fully exposed to the weather, retain to a considerable extent their
colours: this circumstance appears at first surprising, but it is now
known that the colouring principle of the Mytilus is so enduring, that
it is preserved when the shell itself is completely disintegrated. (See
Mr. Lyell “Proofs of a Gradual Rising in Sweden” in the “Philosophical
Transactions” 1835 page 1. See also Mr. Smith of Jordan Hill in the
“Edinburgh New Philosophical Journal” volume 25 page 393.) Most of the
shells are broken; I nowhere found two valves united; the fragments are
not rounded, at least in none of the specimens which I brought home.
With respect to the breadth of the upraised area in an east and west
line, we know from the shells found at the Inner Narrows of the Strait
of Magellan, that the entire width of the plain, although there very
narrow, has been elevated. It is probable that in this southernmost
part of the continent, the movement has extended under the sea far
eastward; for at the Falkland Islands, though I could not find any
shells, the bones of whales have been noticed by several competent
observers, lying on the land at a considerable distance from the sea,
and at the height of some hundred feet above it. (“Voyages of the
‘Adventure’ and ‘Beagle’” volume 2 page 227. And Bougainville’s
“Voyage” tome 1 page 112.) Moreover, we know that in Tierra del Fuego
the boulder formation has been uplifted within the recent period, and a
similar formation occurs on the north-western shores (Byron Sound) of
these islands. (I owe this fact to the kindness of Captain Sulivan,
R.N., a highly competent observer. I mention it more especially, as in
my Paper (page 427) on the Boulder Formation, I have, after having
examined the northern and middle parts of the eastern island, said that
the formation was here wholly absent.) The distance from this point to
the Cordillera of Tierra del Fuego, is 360 miles, which we may take as
the probable width of the recently upraised area. In the latitude of
the R. Santa Cruz, we know from the shells found at the mouth and head,
and in the middle of the valley, that the entire width (about 160
miles) of the surface eastward of the Cordillera has been upraised.
From the slope of the plains, as shown by the course of the rivers, for
several degrees northward of the Santa Cruz, it is probable that the
elevation attested by the shells on the coast has likewise extended to
the Cordillera. When, however, we look as far northward as the
provinces of La Plata, this conclusion would be very hazardous; not
only is the distance from Maldonado (where I found upraised shells) to
the Cordillera great, namely, 760 miles, but at the head of the estuary
of the Plata, a N.N.E. and S.S.W. range of tertiary volcanic rocks has
been observed (This volcanic formation will be described in Chapter IV.
It is not improbable that the height of the upraised shells at the head
of the estuary of the Plata, being greater than at Bahia Blanca or at
San Blas, may be owing to the upheaval of these latter places having
been connected with the distant line of the Cordillera, whilst that of
the provinces of La Plata was in connection with the adjoining tertiary
volcanic axis.), which may well indicate an axis of elevation quite
distinct from that of the Andes. Moreover, in the centre of the Pampas
in the chain of Cordova, severe earthquakes have been felt (See Sir W.
Parish’s work on “La Plata” page 242. For a notice of an earthquake
which drained a lake near Cordova, see also Temple’s “Travels in Peru.”
Sir W. Parish informs me, that a town between Salta and Tucuman (north
of Cordova) was formerly utterly overthrown by an earthquake.); whereas
at Mendoza, at the eastern foot of the Cordillera, only gentle
oscillations, transmitted from the shores of the Pacific, have ever
been experienced. Hence the elevation of the Pampas may be due to
several distinct axes of movement; and we cannot judge, from the
upraised shells round the estuary of the Plata, of the breadth of the
area uplifted within the recent period.
Not only has the above specified long range of coast been elevated
within the recent period, but I think it may be safely inferred from
the similarity in height of the gravel-capped plains at distant points,
that there has been a remarkable degree of equability in the elevatory
process. I may premise, that when I measured the plains, it was simply
to ascertain the heights at which shells occurred; afterwards,
comparing these measurements with some of those made during the Survey,
I was struck with their uniformity, and accordingly tabulated all those
which represented the summit-edges of plains. The extension of the 330
to 355 feet plain is very striking, being found over a space of 500
geographical miles in a north and south line. A table (Table 1) of the
measurements is given below. The angular measurements and all the
estimations (in feet) are by the Officers of the Survey; the
barometrical ones by myself:—
TABLE 1.
Gallegos River to Coy Inlet (partly angular partly estimation) 350
South Side of Santa Cruz (angular and barometric) 355 North Side of
Santa Cruz (angular and barometric) 330 Bird Island, plain opposite to
(angular) 350 Port Desire, plain extending far along coast (barometric)
330 St. George’s Bay, north promontory (angular) 330 Table Land, south
of New Bay (angular) 350
A plain, varying from 245 to 255 feet, seems to extend with much
uniformity from Port Desire to the north of St. George’s Bay, a
distance of 170 miles; and some approximate measurements (in feet),
also given in Table 2 below, indicate the much greater extension of 780
miles:—
TABLE 2.
Coy Inlet, south of (partly angular and partly estimation) 200 to 300
Port Desire (barometric) 245 to 255 C. Blanco (angular) 250 North
Promontory of St. George’s Bay (angular) 250 South of New Bay (angular)
200 to 220 North of S. Josef (estimation) 200 to 300 Plain of Rio Negro
(angular) 200 to 220 Bahia Blanca (estimation) 200 to 300
The extension, moreover, of the 560 to 580, and of the 80 to 100 feet,
plains is remarkable, though somewhat less obvious than in the former
cases. Bearing in mind that I have not picked these measurements out of
a series, but have used all those which represented the edges of
plains, I think it scarcely possible that these coincidences in height
should be accidental. We must therefore conclude that the action,
whatever it may have been, by which these plains have been modelled
into their present forms, has been singularly uniform.
These plains or great terraces, of which three and four often rise like
steps one behind the other, are formed by the denudation of the old
Patagonian tertiary beds, and by the deposition on their surfaces of a
mass of well-rounded gravel, varying, near the coast, from ten to
thirty-five feet in thickness, but increasing in thickness towards the
interior. The gravel is often capped by a thin irregular bed of sandy
earth. The plains slope up, though seldom sensibly to the eye, from the
summit edge of one escarpment to the foot of the next highest one.
Within a distance of 150 miles, between Santa Cruz to Port Desire,
where the plains are particularly well developed, there are at least
seven stages or steps, one above the other. On the three lower ones,
namely, those of 100 feet, 250 feet, and 350 feet in height, existing
littoral shells are abundantly strewed, either on the surface, or
partially embedded in the superficial sandy earth. By whatever action
these three lower plains have been modelled, so undoubtedly have all
the higher ones, up to a height of 950 feet at S. Julian, and of 1,200
feet (by estimation) along St. George’s Bay. I think it will not be
disputed, considering the presence of the upraised marine shells, that
the sea has been the active power during stages of some kind in the
elevatory process.
We will now briefly consider this subject: if we look at the existing
coast-line, the evidence of the great denuding power of the sea is very
distinct; for, from Cape St. Diego, in latitude 54 degrees 30′ to the
mouth of the Rio Negro, in latitude 31 degrees (a length of more than
eight hundred miles), the shore is formed, with singularly few
exceptions, of bold and naked cliffs: in many places the cliffs are
high; thus, south of the Santa Cruz, they are between eight and nine
hundred feet in height, with their horizontal strata abruptly cut off,
showing the immense mass of matter which has been removed. Nearly this
whole line of coast consists of a series of greater or lesser curves,
the horns of which, and likewise certain straight projecting portions,
are formed of hard rocks; hence the concave parts are evidently the
effect and the measure of the denuding action on the softer strata. At
the foot of all the cliffs, the sea shoals very gradually far outwards;
and the bottom, for a space of some miles, everywhere consists of
gravel. I carefully examined the bed of the sea off the Santa Cruz, and
found that its inclination was exactly the same, both in amount and in
its peculiar curvature, with that of the 355 feet plain at this same
place. If, therefore, the coast, with the bed of the adjoining sea,
were now suddenly elevated one or two hundred feet, an inland line of
cliffs, that is an escarpment, would be formed, with a gravel-capped
plain at its foot gently sloping to the sea, and having an inclination
like that of the existing 355 feet plain. From the denuding tendency of
the sea, this newly formed plain would in time be eaten back into a
cliff: and repetitions of this elevatory and denuding process would
produce a series of gravel-capped sloping terraces, rising one above
another, like those fronting the shores of Patagonia.
The chief difficulty (for there are other inconsiderable ones) on this
view, is the fact,—as far as I can trust two continuous lines of
soundings carefully taken between Santa Cruz and the Falkland Islands,
and several scattered observations on this and other coasts,—that the
pebbles at the bottom of the sea QUICKLY and REGULARLY decrease in size
with the increasing depth and distance from the shore, whereas in the
gravel on the sloping plains, no such decrease in size was perceptible.
Table 3 below gives the average result of many soundings off the Santa
Cruz:— TABLE 3.
Under two miles from the shore, many of the pebbles were of large size,
mingled with some small ones.
Column 1. Distance in miles from the shore.
Column 2. Depth in fathoms.
Column 3. Size of Pebbles.
I particularly attended to the size of the pebbles on the 355 feet
Santa Cruz plain, and I noticed that on the summit-edge of the present
sea cliffs many were as large as half a man’s head; and in crossing
from these cliffs to the foot of the next highest escarpment, a
distance of six miles, I could not observe any increase in their size.
We shall presently see that the theory of a slow and almost insensible
rise of the land, will explain all the facts connected with the
gravel-capped terraces, better than the theory of sudden elevations of
from one to two hundred feet.
M. d’Orbigny has argued, from the upraised shells at San Blas being
embedded in the positions in which they lived, and from the valves of
the Azara labiata high on the banks of the Parana being united and
unrolled, that the elevation of Northern Patagonia and of La Plata must
have been sudden; for he thinks, if it had been gradual, these shells
would all have been rolled on successive beach-lines. But in PROTECTED
bays, such as in that of Bahia Blanca, wherever the sea is accumulating
extensive mud-banks, or where the winds quietly heap up sand-dunes,
beds of shells might assuredly be preserved buried in the positions in
which they had lived, even whilst the land retained the same level;
any, the smallest, amount of elevation would directly aid in their
preservation. I saw a multitude of spots in Bahia Blanca where this
might have been effected; and at Maldonado it almost certainly has been
effected. In speaking of the elevation of the land having been slow, I
do not wish to exclude the small starts which accompany earthquakes, as
on the coast of Chile; and by such movements beds of shells might
easily be uplifted, even in positions exposed to a heavy surf, without
undergoing any attrition: for instance, in 1835, a rocky flat off the
island of Santa Maria was at one blow upheaved above high-water mark,
and was left covered with gaping and putrefying mussel-shells, still
attached to the bed on which they had lived. If M. d’Orbigny had been
aware of the many long parallel lines of sand-hillocks, with infinitely
numerous shells of the Mactra and Venus, at a low level near the
Uruguay; if he had seen at Bahia Blanca the immense sand-dunes, with
water-worn pebbles of pumice, ranging in parallel lines, one behind the
other, up a height of at least 120 feet; if he had seen the sand-dunes,
with the countless Paludestrinas, on the low plain near the Fort at
this place, and that long line on the edge of the cliff, sixty feet
higher up; if he had crossed that long and great belt of parallel
sand-dunes, eight miles in width, standing at the height of from forty
to fifty feet above the Colorado, where sand could not now collect,—I
cannot believe he would have thought that the elevation of this great
district had been sudden. Certainly the sand-dunes (especially when
abounding with shells), which stand in ranges at so many different
levels, must all have required long time for their accumulation; and
hence I do not doubt that the last 100 feet of elevation of La Plata
and Northern Patagonia has been exceedingly slow.
If we extend this conclusion to Central and Southern Patagonia, the
inclination of the successively rising gravel-capped plains can be
explained quite as well, as by the more obvious view already given of a
few comparatively great and sudden elevations; in either case we must
admit long periods of rest, during which the sea ate deeply into the
land. Let us suppose the present coast to rise at a nearly equable,
slow rate, yet sufficiently quick to prevent the waves quite removing
each part as soon as brought up; in this case every portion of the
present bed of the sea will successively form a beach-line, and from
being exposed to a like action will be similarly affected. It cannot
matter to what height the tides rise, even if to forty feet as at Santa
Cruz, for they will act with equal force and in like manner on each
successive line. Hence there is no difficulty in the fact of the 355
feet plain at Santa Cruz sloping up 108 feet to the foot of the next
highest escarpment, and yet having no marks of any one particular
beach-line on it; for the whole surface on this view has been a beach.
I cannot pretend to follow out the precise action of the tidal-waves
during a rise of the land, slow, yet sufficiently quick to prevent or
check denudation: but if it be analogous to what takes place on
protected parts of the present coast, where gravel is now accumulating
in large quantities, an inclined surface, thickly capped by
well-rounded pebbles of about the same size, would be ultimately left.
(On the eastern side of Chiloe, which island we shall see in the next
chapter is now rising, I observed that all the beaches and extensive
tidal-flats were formed of shingle.) On the gravel now accumulating,
the waves, aided by the wind, sometimes throw up a thin covering of
sand, together with the common coast-shells. Shells thus cast up by
gales, would, during an elevatory period, never again be touched by the
sea. Hence, on this view of a slow and gradual rising of the land,
interrupted by periods of rest and denudation, we can understand the
pebbles being of about the same size over the entire width of the
step-like plains,—the occasional thin covering of sandy earth,—and the
presence of broken, unrolled fragments of those shells, which now live
exclusively near the coast.
A SUMMARY OF RESULTS.
It may be concluded that the coast on this side of the continent, for a
space of at least 1,180 miles, has been elevated to a height of 100
feet in La Plata, and of 400 feet in Southern Patagonia, within the
period of existing shells, but not of existing mammifers. That in La
Plata the elevation has been very slowly effected: that in Patagonia
the movement may have been by considerable starts, but much more
probably slow and quiet. In either case, there have been long
intervening periods of comparative rest, during which the sea corroded
deeply, as it is still corroding, into the land. (I say COMPARATIVE and
not ABSOLUTE rest, because the sea acts, as we have seen, with great
denuding power on this whole line of coast; and therefore, during an
elevation of the land, if excessively slow (and of course during a
subsidence of the land), it is quite possible that lines of cliff might
be formed.) That the periods of denudation and elevation were
contemporaneous and equable over great spaces of coast, as shown by the
equable heights of the plains; that there have been at least eight
periods of denudation, and that the land, up to a height of from 950 to
1,200 feet, has been similarly modelled and affected: that the area
elevated, in the southernmost part of the continent, extended in
breadth to the Cordillera, and probably seaward to the Falkland
Islands; that northward, in La Plata, the breadth is unknown, there
having been probably more than one axis of elevation; and finally,
that, anterior to the elevation attested by these upraised shells, the
land was divided by a Strait where the River Santa Cruz now flows, and
that further southward there were other sea-straits, since closed. I
may add, that at Santa Cruz, in latitude 50 degrees S., the plains have
been uplifted at least 1,400 feet, since the period when gigantic
boulders were transported between sixty and seventy miles from their
parent rock, on floating icebergs.
Lastly, considering the great upward movements which this long line of
coast has undergone, and the proximity of its southern half to the
volcanic axis of the Cordillera, it is highly remarkable that in the
many fine sections exposed in the Pampean, Patagonian tertiary, and
Boulder formations, I nowhere observed the smallest fault or abrupt
curvature in the strata.
GRAVEL FORMATION OF PATAGONIA.
I will here describe in more detail than has been as yet incidentally
done, the nature, origin, and extent of the great shingle covering of
Patagonia: but I do not mean to affirm that all of this shingle,
especially that on the higher plains, belongs to the recent period. A
thin bed of sandy earth, with small pebbles of various porphyries and
of quartz, covering a low plain on the north side of the Rio Colorado,
is the extreme northern limit of this formation. These little pebbles
have probably been derived from the denudation of a more regular bed of
gravel, capping the old tertiary sandstone plateau of the Rio Negro.
The gravel-bed near the Rio Negro is, on an average, about ten or
twelve feet in thickness; and the pebbles are larger than on the
northern side of the Colorado, being from one or two inches in
diameter, and composed chiefly of rather dark-tinted porphyries.
Amongst them I here first noticed a variety often to be referred to,
namely, a peculiar gallstone-yellow siliceous porphyry, frequently, but
not invariably, containing grains of quartz. The pebbles are embedded
in a white, gritty, calcareous matrix, very like mortar, sometimes
merely coating with a whitewash the separate stones, and sometimes
forming the greater part of the mass. In one place I saw in the gravel
concretionary nodules (not rounded) of crystallised gypsum, some as
large as a man’s head. I traced this bed for forty-five miles inland,
and was assured that it extended far into the interior. As the surface
of the calcareo- argillaceous plain of Pampean formation, on the
northern side of the wide valley of the Colorado, stands at about the
same height with the mortar- like cemented gravel capping the sandstone
on the southern side, it is probable, considering the apparent
equability of the subterranean movements along this side of America,
that this gravel of the Rio Negro and the upper beds of the Pampean
formation northward of the Colorado, are of nearly contemporaneous
origin, and that the calcareous matter has been derived from the same
source.
Southward of the Rio Negro, the cliffs along the great bay of S.
Antonio are capped with gravel: at San Josef, I found that the pebbles
closely resembled those on the plain of the Rio Negro, but that they
were not cemented by calcareous matter. Between San Josef and Port
Desire, I was assured by the Officers of the Survey that the whole face
of the country is coated with gravel. At Port Desire and over a space
of twenty-five miles inland, on the three step-formed plains and in the
valleys, I everywhere passed over gravel which, where thickest, was
between thirty and forty feet. Here, as in other parts of Patagonia,
the gravel, or its sandy covering, was, as we have seen, often strewed
with recent marine shells. The sandy covering sometimes fills up
furrows in the gravel, as does the gravel in the underlying tertiary
formations. The pebbles are frequently whitewashed and even cemented
together by a peculiar, white, friable, aluminous, fusible substance,
which I believe is decomposed feldspar. At Port Desire, the gravel
rested sometimes on the basal formation of porphyry, and sometimes on
the upper or the lower denuded tertiary strata. It is remarkable that
most of the porphyritic pebbles differ from those varieties of porphyry
which occur here abundantly in situ. The peculiar gallstone-yellow
variety was common, but less numerous than at Port S. Julian, where it
formed nearly one-third of the mass of the gravel; the remaining part
there consisting of pale grey and greenish porphyries with many
crystals of feldspar. At Port S. Julian, I ascended one of the flat-
topped hills, the denuded remnant of the highest plain, and found it,
at the height of 950 feet, capped with the usual bed of gravel.
Near the mouth of the Santa Cruz, the bed of gravel on the 355 feet
plain is from twenty to about thirty-five feet in thickness. The
pebbles vary from minute ones to the size of a hen’s egg, and even to
that of half a man’s head; they consist of paler varieties of porphyry
than those found further northward, and there are fewer of the
gallstone-yellow kind; pebbles of compact black clay-slate were here
first observed. The gravel, as we have seen, covers the step-formed
plains at the mouth, head, and on the sides of the great valley of the
Santa Cruz. At a distance of 110 miles from the coast, the plain has
risen to the height of 1,416 feet above the sea; and the gravel, with
the associated great boulder formation, has attained a thickness of 212
feet. The plain, apparently with its usual gravel covering, slopes up
to the foot of the Cordillera to the height of between 3,200 and 3,300
feet. In ascending the valley, the gravel gradually becomes entirely
altered in character: high up, we have pebbles of crystalline
feldspathic rocks, compact clay-slate, quartzose schists, and
pale-coloured porphyries; these rocks, judging both from the gigantic
boulders in the surface and from some small pebbles embedded beneath
700 feet in thickness of the old tertiary strata, are the prevailing
kinds in this part of the Cordillera; pebbles of basalt from the
neighbouring streams of basaltic lava are also numerous; there are few
or none of the reddish or of the gallstone-yellow porphyries so common
near the coast. Hence the pebbles on the 350 feet plain at the mouth of
the Santa Cruz cannot have been derived (with the exception of those of
compact clay- slate, which, however, may equally well have come from
the south) from the Cordillera in this latitude; but probably, in chief
part, from farther north.
Southward of the Santa Cruz, the gravel may be seen continuously
capping the great 840 feet plain: at the Rio Gallegos, where this plain
is succeeded by a lower one, there is, as I am informed by Captain
Sulivan, an irregular covering of gravel from ten to twelve feet in
thickness over the whole country. The district on each side of the
Strait of Magellan is covered up either with gravel or the boulder
formation: it was interesting to observe the marked difference between
the perfectly rounded state of the pebbles in the great shingle
formation of Patagonia, and the more or less angular fragments in the
boulder formation. The pebbles and fragments near the Strait of
Magellan nearly all belong to rocks known to occur in Fuegia. I was
therefore much surprised in dredging south of the Strait to find, in
latitude 54 degrees 10′ south, many pebbles of the gallstone-yellow
siliceous porphyry; I procured others from a great depth off Staten
Island, and others were brought me from the western extremity of the
Falkland Islands. (At my request, Mr. Kent collected for me a bag of
pebbles from the beach of White Rock harbour, in the northern part of
the sound, between the two Falkland Islands. Out of these well-rounded
pebbles, varying in size from a walnut to a hen’s egg, with some
larger, thirty-eight evidently belonged to the rocks of these islands;
twenty-six were similar to the pebbles of porphyry found on the
Patagonian plains, which rocks do not exist in situ in the Falklands;
one pebble belonged to the peculiar yellow siliceous porphyry; thirty
were of doubtful origin.) The distribution of the pebbles of this
peculiar porphyry, which I venture to affirm is not found in situ
either in Fuegia, the Falkland Islands, or on the coast of Patagonia,
is very remarkable, for they are found over a space of 840 miles in a
north and south line, and at the Falklands, 300 miles eastward of the
coast of Patagonia. Their occurrence in Fuegia and the Falklands may,
however, perhaps be due to the same ice-agency by which the boulders
have been there transported.
We have seen that porphyritic pebbles of a small size are first met
with on the northern side of the Rio Colorado, the bed becoming well
developed near the Rio Negro: from this latter point I have every
reason to believe that the gravel extends uninterruptedly over the
plains and valleys of Patagonia for at least 630 nautical miles
southward to the Rio Gallegos. From the slope of the plains, from the
nature of the pebbles, from their extension at the Rio Negro far into
the interior, and at the Santa Cruz close up to the Cordillera, I think
it highly probable that the whole breadth of Patagonia is thus covered.
If so, the average width of the bed must be about two hundred miles.
Near the coast the gravel is generally from ten to thirty feet in
thickness; and as in the valley of Santa Cruz it attains, at some
distance from the Cordillera, a thickness of 214 feet, we may, I think,
safely assume its average thickness over the whole area of 630 by 200
miles, at fifty feet!
The transportal and origin of this vast bed of pebbles is an
interesting problem. From the manner in which they cap the step-formed
plains, worn by the sea within the period of existing shells, their
deposition, at least on the plains up to a height of 400 feet, must
have been a recent geological event. From the form of the continent, we
may feel sure that they have come from the westward, probably, in chief
part from the Cordillera, but, perhaps, partly from unknown rocky
ridges in the central districts of Patagonia. That the pebbles have not
been transported by rivers, from the interior towards the coast, we may
conclude from the fewness and smallness of the streams of Patagonia:
moreover, in the case of the one great and rapid river of Santa Cruz,
we have good evidence that its transporting power is very trifling.
This river is from two to three hundred yards in width, about seventeen
feet deep in its middle, and runs with a singular degree of uniformity
five knots an hour, with no lakes and scarcely any still reaches:
nevertheless, to give one instance of its small transporting power,
upon careful examination, pebbles of compact basalt could not be found
in the bed of the river at a greater distance than ten miles below the
point where the stream rushes over the debris of the great basaltic
cliffs forming its shore: fragments of the CELLULAR varieties have been
washed down twice or thrice as far. That the pebbles in Central and
Northern Patagonia have not been transported by ice-agency, as seems to
have been the case to a considerable extent farther south, and likewise
in the northern hemisphere, we may conclude, from the absence of all
angular fragments in the gravel, and from the complete contrast in many
other respects between the shingle and neighbouring boulder formation.
Looking to the gravel on any one of the step-formed plains, I cannot
doubt, from the several reasons assigned in this chapter, that it has
been spread out and leveled by the long-continued action of the sea,
probably during the slow rise of the land. The smooth and perfectly
rounded condition of the innumerable pebbles alone would prove
long-continued action. But how the whole mass of shingle on the
coast-plains has been transported from the mountains of the interior,
is another and more difficult question. The following considerations,
however, show that the sea by its ordinary action has considerable
power in distributing pebbles. Table 3 above shows how very uniformly
and gradually the pebbles decrease in size with the gradually seaward
increasing depth and distance. (I may mention, that at the distance of
150 miles from the Patagonian shore I carefully examined the minute
rounded particles in the sand, and found them to be fusible like the
porphyries of the great shingle bed. I could even distinguish particles
of the gallstone-yellow porphyry. It was interesting to notice how
gradually the particles of white quartz increased, as we approached the
Falkland Islands, which are thus constituted. In the whole line of
soundings between these islands and the coast of Patagonia dead or
living organic remains were most rare. On the relations between the
depth of water and the nature of the bottom, see Martin White on
“Soundings in the Channel” pages 4, 6, 175; also Captain Beechey’s
“Voyage to the Pacific” chapter 18.) A series of this kind irresistibly
leads to the conclusion, that the sea has the power of sifting and
distributing the loose matter on its bottom. According to Martin White,
the bed of the British Channel is disturbed during gales at depths of
sixty-three and sixty-seven fathoms, and at thirty fathoms, shingle and
fragments of shells are often deposited, afterwards to be carried away
again. (“Soundings in the Channel” pages 4, 166. M. Siau states
(“Edinburgh New Philosophical Journal” volume 31 page 246), that he
found the sediment, at a depth of 188 metres, arranged in ripples of
different degrees of fineness. There are some excellent discussions on
this and allied subjects in Sir H. De la Beche’s “Theoretical
Researches.”) Groundswells, which are believed to be caused by distant
gales, seem especially to affect the bottom: at such times, according
to Sir R. Schomburgk, the sea to a great distance round the West Indian
Islands, at depths from five to fifteen fathoms, becomes discoloured,
and even the anchors of vessels have been moved. (“Journal of Royal
Geographical Society” volume 5 page 25. It appears from Mr. Scott
Russell’s investigations (see Mr. Murchison’s “Anniversary Address
Geological Society” 1843 page 40), that in waves of translation the
motion of the particles of water is nearly as great at the bottom as at
the top.) There are, however, some difficulties in understanding how
the sea can transport pebbles lying at the bottom, for, from
experiments instituted on the power of running water, it would appear
that the currents of the sea have not sufficient velocity to move
stones of even moderate size: moreover, I have repeatedly found in the
most exposed situations that the pebbles which lie at the bottom are
encrusted with full-grown living corallines, furnished with the most
delicate, yet unbroken spines: for instance, in ten fathoms water off
the mouth of the Santa Cruz, many pebbles, under half an inch in
diameter, were thus coated with Flustracean zoophytes. (A pebble, one
and a half inch square and half an inch thick, was given me, dredged up
from twenty-seven fathoms depth off the western end of the Falkland
Islands, where the sea is remarkably stormy, and subject to violent
tides. This pebble was encrusted on all sides by a delicate living
coralline. I have seen many pebbles from depths between forty and
seventy fathoms thus encrusted; one from the latter depth off Cape
Horn.) Hence we must conclude that these pebbles are not often
violently disturbed: it should, however, be borne in mind that the
growth of corallines is rapid. The view, propounded by Professor
Playfair, will, I believe, explain this apparent difficulty,—namely,
that from the undulations of the sea TENDING to lift up and down
pebbles or other loose bodies at the bottom, such are liable, when thus
quite or partially raised, to be moved even by a very small force, a
little onwards. We can thus understand how oceanic or tidal currents of
no great strength, or that recoil movement of the bottom-water near the
land, called by sailors the “undertow” (which I presume must extend out
seaward as far as the BREAKING waves impel the surface-water towards
the beach), may gain the power during storms of sifting and
distributing pebbles even of considerable size, and yet without so
violently disturbing them as to injure the encrusting corallines. (I
may take this opportunity of remarking on a singular, but very common
character in the form of the bottom, in the creeks which deeply
penetrate the western shores of Tierra del Fuego; namely, that they are
almost invariably much shallower close to the open sea at their mouths
than inland. Thus, Cook, in entering Christmas Sound, first had
soundings in thirty-seven fathoms, then in fifty, then in sixty, and a
little farther in no bottom with 170 fathoms. The sealers are so
familiar with this fact, that they always look out for anchorage near
the entrances of the creeks. See, also, on this subject, the “Voyages
of the ‘Adventure’ and ‘Beagle’” volume 1 page 375 and “Appendix” page
313. This Shoalness of the sea- channels near their entrances probably
results from the quantity of sediment formed by the wear and tear of
the outer rocks exposed to the full force of the open sea. I have no
doubt that many lakes, for instance in Scotland, which are very deep
within, and are separated from the sea apparently only by a tract of
detritus, were originally sea-channels with banks of this nature near
their mouths, which have since been upheaved.)
The sea acts in another and distinct manner in the distribution of
pebbles, namely by the waves on the beach. Mr. Palmer, in his excellent
memoir on this subject, has shown that vast masses of shingle travel
with surprising quickness along lines of coast, according to the
direction with which the waves break on the beach and that this is
determined by the prevailing direction of the winds. (“Philosophical
Transactions” 1834 page 576.) This agency must be powerful in mingling
together and disseminating pebbles derived from different sources: we
may, perhaps, thus understand the wide distribution of the
gallstone-yellow porphyry; and likewise, perhaps, the great difference
in the nature of the pebbles at the mouth of the Santa Cruz from those
in the same latitude at the head of the valley.
I will not pretend to assign to these several and complicated agencies
their shares in the distribution of the Patagonian shingle: but from
the several considerations given in this chapter, and I may add, from
the frequency of a capping of gravel on tertiary deposits in all parts
of the world, as I have myself observed and seen stated in the works of
various authors, I cannot doubt that the power of widely dispersing
gravel is an ordinary contingent on the action of the sea; and that
even in the case of the great Patagonian shingle-bed we have no
occasion to call in the aid of debacles. I at one time imagined that
perhaps an immense accumulation of shingle had originally been
collected at the foot of the Cordillera; and that this accumulation,
when upraised above the level of the sea, had been eaten into and
partially spread out (as off the present line of coast); and that the
newly-spread out bed had in its turn been upraised, eaten into, and
re-spread out; and so onwards, until the shingle, which was first
accumulated in great thickness at the foot of the Cordillera, had
reached in thinner beds its present extension. By whatever means the
gravel formation of Patagonia may have been distributed, the vastness
of its area, its thickness, its superficial position, its recent
origin, and the great degree of similarity in the nature of its
pebbles, all appear to me well deserving the attention of geologists,
in relation to the origin of the widely-spread beds of conglomerate
belonging to past epochs.
FORMATION OF CLIFFS.
(DIAGRAM 7.—SECTION OF COAST-CLIFFS AND BOTTOM OF SEA, OFF THE ISLAND
OF ST. HELENA.
Height in feet above sea level.
Depths in fathoms.
Vertical and horizontal scale, two inches to a nautical mile. The point
marked 1,600 feet is at the foot of High Knoll; point marked 510 feet
is on the edge of Ladder Hill. The strata consist of basaltic streams.
A Section left to right:
Height at the foot of High Knoll: 1,600 at top of strata.
Height on the edge of Ladder Hill: 510 at top of strata.
Bottom at coast rocky only to a depth of five or six fathoms.
30 fathoms: bottom mud and sand.
100 fathoms sloping more sharply to 250 fathoms.)
When viewing the sea-worn cliffs of Patagonia, in some parts between
eight hundred and nine hundred feet in height, and formed of horizontal
tertiary strata, which must once have extended far seaward—or again,
when viewing the lofty cliffs round many volcanic islands, in which the
gentle inclination of the lava-streams indicates the former extension
of the land, a difficulty often occurred to me, namely, how the strata
could possibly have been removed by the action of the sea at a
considerable depth beneath its surface. The section in Diagram 7, which
represents the general form of the land on the northern and leeward
side of St. Helena (taken from Mr. Seale’s large model and various
measurements), and of the bottom of the adjoining sea (taken chiefly
from Captain Austin’s survey and some old charts), will show the nature
of this difficulty.
If, as seems probable, the basaltic streams were originally prolonged
with nearly their present inclination, they must, as shown by the
dotted line in the section, once have extended at least to a point, now
covered by the sea to a depth of nearly thirty fathoms: but I have
every reason to believe they extended considerably further, for the
inclination of the streams is less near the coast than further inland.
It should also be observed, that other sections on the coast of this
island would have given far more striking results, but I had not the
exact measurements; thus, on the windward side, the cliffs are about
two thousand feet in height and the cut-off lava streams very gently
inclined, and the bottom of the sea has nearly a similar slope all
round the island. How, then, has all the hard basaltic rock, which once
extended beneath the surface of the sea, been worn away? According to
Captain Austin, the bottom is uneven and rocky only to that very small
distance from the beach within which the depth is from five to six
fathoms; outside this line, to a depth of about one hundred fathoms,
the bottom is smooth, gently inclined, and formed of mud and sand;
outside the one hundred fathoms, it plunges suddenly into unfathomable
depths, as is so very commonly the case on all coasts where sediment is
accumulating. At greater depths than the five or six fathoms, it seems
impossible, under existing circumstances, that the sea can both have
worn away hard rock, in parts to a thickness of at least 150 feet, and
have deposited a smooth bed of fine sediment. Now, if we had any reason
to suppose that St. Helena had, during a long period, gone on slowly
subsiding, every difficulty would be removed: for looking at the
diagram, and imagining a fresh amount of subsidence, we can see that
the waves would then act on the coast-cliffs with fresh and unimpaired
vigour, whilst the rocky ledge near the beach would be carried down to
that depth, at which sand and mud would be deposited on its bare and
uneven surface: after the formation near the shore of a new rocky
shoal, fresh subsidence would carry it down and allow it to be smoothly
covered up. But in the case of the many cliff-bounded islands, for
instance in some of the Canary Islands and of Madeira, round which the
inclination of the strata shows that the land once extended far into
the depths of the sea, where there is no apparent means of hard rock
being worn away—are we to suppose that all these islands have slowly
subsided? Madeira, I may remark, has, according to Mr. Smith of Jordan
Hill, subsided. Are we to extend this conclusion to the high, cliff-
bound, horizontally stratified shores of Patagonia, off which, though
the water is not deep even at the distance of several miles, yet the
smooth bottom of pebbles gradually decreasing in size with the
increasing depth, and derived from a foreign source, seem to declare
that the sea is now a depositing and not a corroding agent? I am much
inclined to suspect, that we shall hereafter find in all such cases,
that the land with the adjoining bed of the sea has in truth subsided:
the time will, I believe, come, when geologists will consider it as
improbable, that the land should have retained the same level during a
whole geological period, as that the atmosphere should have remained
absolutely calm during an entire season.
CHAPTER II.
ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.
Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions
of the inhabitants on this subject.—Concepcion, earthquake and
elevation of.—VALPARAISO, great elevation of, upraised shells, earth of
marine origin, gradual rise of the land within the historical
period.—COQUIMBO, elevation of, in recent times; terraces of marine
origin, their inclination, their escarpments not horizontal.—Guasco,
gravel terraces of.—Copiapo.—PERU.—Upraised shells of Cobija, Iquique,
and Arica.—Lima, shell-beds and sea-beach on San Lorenzo, human
remains, fossil earthenware, earthquake debacle, recent subsidence.—On
the decay of upraised shells.—General summary.
Commencing at the south and proceeding northward, the first place at
which I landed, was at Cape Tres Montes, in latitude 46 degrees 35′.
Here, on the shores of Christmas Cove, I observed in several places a
beach of pebbles with recent shells, about twenty feet above high-water
mark. Southward of Tres Montes (between latitude 47 and 48 degrees),
Byron remarks, “We thought it very strange, that upon the summits of
the highest hills were found beds of shells, a foot or two thick.”
(“Narrative of the Loss of the ‘Wager’.”) In the Chonos Archipelago,
the island of Lemus (latitude 44 degrees 30′) was, according to M.
Coste, suddenly elevated eight feet, during the earthquake of 1829: he
adds, “Des roches jadis toujours couvertes par la mer, restant
aujourd’hui constamment decouvertes.” (“Comptes Rendus” October 1838
page 706.) In other parts of this archipelago, I observed two terraces
of gravel, abutting to the foot of each other: at Lowe’s Harbour (43
degrees 48′), under a great mass of the boulder formation, about three
hundred feet in thickness, I found a layer of sand, with numerous
comminuted fragments of sea-shells, having a fresh aspect, but too
small to be identified.
THE ISLAND OF CHILOE.
The evidence of recent elevation is here more satisfactory. The bay of
San Carlos is in most parts bounded by precipitous cliffs from about
ten to forty feet in height, their bases being separated from the
present line of tidal action by a talus, a few feet in height, covered
with vegetation. In one sheltered creek (west of P. Arena), instead of
a loose talus, there was a bare sloping bank of tertiary mudstone,
perforated, above the line of the highest tides, by numerous shells of
a Pholas now common in the harbour. The upper extremities of these
shells, standing upright in their holes with grass growing out of them,
were abraded about a quarter of an inch, to the same level with the
surrounding worn strata. In other parts, I observed (as at Pudeto) a
great beach, formed of comminuted shells, twenty feet above the present
shore. In other parts again, there were small caves worn into the foot
of the low cliffs, and protected from the waves by the talus with its
vegetation: one such cave, which I examined, had its mouth about twenty
feet, and its bottom, which was filled with sand containing fragments
of shells and legs of crabs, from eight to ten feet above high-water
mark. From these several facts, and from the appearance of the upraised
shells, I inferred that the elevation had been quite recent; and on
inquiring from Mr. Williams, the Portmaster, he told me he was
convinced that the land had risen, or the sea fallen, four feet within
the last four years. During this period, there had been one severe
earthquake, but no particular change of level was then observed; from
the habits of the people who all keep boats in the protected creeks, it
is absolutely impossible that a rise of four feet could have taken
place suddenly and been unperceived. Mr. Williams believes that the
change has been quite gradual. Without the elevatory movement continues
at a quick rate, there can be no doubt that the sea will soon destroy
the talus of earth at the foot of the cliffs round the bay, and will
then reach its former lateral extension, but not of course its former
level: some of the inhabitants assured me that one such talus, with a
footpath on it, was even already sensibly decreasing in width.
I received several accounts of beds of shells, existing at considerable
heights in the inland parts of Chiloe; and to one of these, near
Catiman, I was guided by a countryman. Here, on the south side of the
peninsula of Lacuy, there was an immense bed of the Venus costellata
and of an oyster, lying on the summit-edge of a piece of tableland, 350
feet (by the barometer) above the level of the sea. The shells were
closely packed together, embedded in and covered by a very black, damp,
peaty mould, two or three feet in thickness, out of which a forest of
great trees was growing. Considering the nature and dampness of this
peaty soil, it is surprising that the fine ridges on the outside of the
Venus are perfectly preserved, though all the shells have a blackened
appearance. I did not doubt that the black soil, which when dry, cakes
hard, was entirely of terrestrial origin, but on examining it under the
microscope, I found many very minute rounded fragments of shells,
amongst which I could distinguish bits of Serpulae and mussels. The
Venus costellata, and the Ostrea (O. edulis, according to Captain King)
are now the commonest shells in the adjoining bays. In a bed of shells,
a few feet below the 350 feet bed, I found a horn of the little Cervus
humilis, which now inhabits Chiloe.
The eastern or inland side of Chiloe, with its many adjacent islets,
consists of tertiary and boulder deposits, worn into irregular plains
capped by gravel. Near Castro, and for ten miles southward, and on the
islet of Lemuy, I found the surface of the ground to a height of
between twenty and thirty feet above high-water mark, and in several
places apparently up to fifty feet, thickly coated by much comminuted
shells, chiefly of the Venus costellata and Mytilus Chiloensis; the
species now most abundant on this line of coast. As the inhabitants
carry immense numbers of these shells inland, the continuity of the bed
at the same height was often the only means of recognising its natural
origin. Near Castro, on each side of the creek and rivulet of the
Gamboa, three distinct terraces are seen: the lowest was estimated at
about one hundred and fifty feet in height, and the highest at about
five hundred feet, with the country irregularly rising behind it;
obscure traces, also, of these same terraces could be seen along other
parts of the coast. There can be no doubt that their three escarpments
record pauses in the elevation of the island. I may remark that several
promontories have the word Huapi, which signifies in the Indian tongue,
island, appended to them, such as Huapilinao, Huapilacuy, Caucahuapi,
etc.; and these, according to Indian traditions, once existed as
islands. In the same manner the term Pulo in Sumatra is appended to the
names of promontories, traditionally said to have been islands
(Marsden’s “Sumatra” page 31.); in Sumatra, as in Chiloe, there are
upraised recent shells. The Bay of Carelmapu, on the mainland north of
Chiloe, according to Aguerros, was in 1643 a good harbour (“Descripcion
Hist. de la Provincia de Chiloe” page 78. From the account given by the
old Spanish writers, it would appear that several other harbours,
between this point and Concepcion, were formerly much deeper than they
now are.); it is now quite useless, except for boats.
VALDIVIA.
I did not observe here any distinct proofs of recent elevation; but in
a bed of very soft sandstone, forming a fringe-like plain, about sixty
feet in height, round the hills of mica-slate, there are shells of
Mytilus, Crepidula, Solen, Novaculina, and Cytheraea, too imperfect to
be specifically recognised. At Imperial, seventy miles north of
Valdivia, Aguerros states that there are large beds of shells, at a
considerable distance from the coast, which are burnt for lime. (Ibid
page 25.) The island of Mocha, lying a little north of Imperial, was
uplifted two feet, during the earthquake of 1835. (“Voyages of
‘Adventure’ and ‘Beagle’” volume 2 page 415.)
CONCEPCION.
I cannot add anything to the excellent account by Captain Fitzroy of
the elevation of the land at this place, which accompanied the
earthquake of 1835. (Ibid volume 2 page 412 et seq. In volume 5 page
601 of the “Geological Transactions” I have given an account of the
remarkable volcanic phenomena, which accompanied this earthquake. These
phenomena appear to me to prove that the action, by which large tracts
of land are uplifted, and by which volcanic eruptions are produced, is
in every respect identical.) I will only recall to the recollection of
geologists, that the southern end of the island of St. Mary was
uplifted eight feet, the central part nine, and the northern end ten
feet; and the whole island more than the surrounding districts. Great
beds of mussels, patellae, and chitons still adhering to the rocks were
upraised above high-water mark; and some acres of a rocky flat, which
was formerly always covered by the sea, was left standing dry, and
exhaled an offensive smell, from the many attached and putrefying
shells. It appears from the researches of Captain Fitzroy that both the
island of St. Mary and Concepcion (which was uplifted only four or five
feet) in the course of some weeks subsided, and lost part of their
first elevation. I will only add as a lesson of caution, that round the
sandy shores of the great Bay of Concepcion, it was most difficult,
owing to the obliterating effects of the great accompanying wave, to
recognise any distinct evidence of this considerable upheaval; one spot
must be excepted, where there was a detached rock which before the
earthquake had always been covered by the sea, but afterwards was left
uncovered.
On the island of Quiriquina (in the Bay of Concepcion), I found, at an
estimated height of four hundred feet, extensive layers of shells,
mostly comminuted, but some perfectly preserved and closely packed in
black vegetable mould; they consisted of Concholepas, Fissurella,
Mytilus, Trochus, and Balanus. Some of these layers of shells rested on
a thick bed of bright-red, dry, friable earth, capping the surface of
the tertiary sandstone, and extending, as I observed whilst sailing
along the coast, for 150 miles southward: at Valparaiso, we shall
presently see that a similar red earthy mass, though quite like
terrestrial mould, is really in chief part of recent marine origin. On
the flanks of this island of Quiriquina, at a less height than the 400
feet, there were spaces several feet square, thickly strewed with
fragments of similar shells. During a subsequent visit of the “Beagle”
to Concepcion, Mr. Kent, the assistant-surgeon, was so kind as to make
for me some measurements with the barometer: he found many marine
remains along the shores of the whole bay, at a height of about twenty
feet; and from the hill of Sentinella behind Talcahuano, at the height
of 160 feet, he collected numerous shells, packed together close
beneath the surface in black earth, consisting of two species of
Mytilus, two of Crepidula, one of Concholepas, of Fissurella, Venus,
Mactra, Turbo, Monoceros, and the Balanus psittacus. These shells were
bleached, and within some of the Balani other Balani were growing,
showing that they must have long lain dead in the sea. The above
species I compared with living ones from the bay, and found them
identical; but having since lost the specimens, I cannot give their
names: this is of little importance, as Mr. Broderip has examined a
similar collection, made during Captain Beechey’s expedition, and
ascertained that they consisted of ten recent species, associated with
fragments of Echini, crabs, and Flustrae; some of these remains were
estimated by Lieutenant Belcher to lie at the height of nearly a
thousand feet above the level of the sea. (“Zoology of Captain
Beechey’s Voyage” page 162.) In some places round the bay, Mr. Kent
observed that there were beds formed exclusively of the Mytilus
Chiloensis: this species now lives in parts never uncovered by the
tides. At considerable heights, Mr. Kent found only a few shells; but
from the summit of one hill, 625 feet high, he brought me specimens of
the Concholepas, Mytilus Chiloensis, and a Turbo. These shells were
softer and more brittle than those from the height of 164 feet; and
these latter had obviously a much more ancient appearance than the same
species from the height of only twenty feet.
COAST NORTH OF CONCEPCION.
The first point examined was at the mouth of the Rapel (160 miles north
of Concepcion and sixty miles south of Valparaiso), where I observed a
few shells at the height of 100 feet, and some barnacles adhering to
the rocks three or four feet above the highest tides: M. Gay found here
recent shells at the distance of two leagues from the shore. (“Annales
des Scienc. Nat.” Avril 1833.) Inland there are some wide,
gravel-capped plains, intersected by many broad, flat-bottomed valleys
(now carrying insignificant streamlets), with their sides cut into
successive wall-like escarpments, rising one above another, and in many
places, according to M. Gay, worn into caves. The one cave (C. del
Obispo) which I examined, resembled those formed on many sea-coasts,
with its bottom filled with shingle. These inland plains, instead of
sloping towards the coast, are inclined in an opposite direction
towards the Cordillera, like the successively rising terraces on the
inland or eastern side of Chiloe: some points of granite, which project
through the plains near the coast, no doubt once formed a chain of
outlying islands, on the inland shores of which the plains were
accumulated. At Bucalemu, a few miles northward of the Rapel, I
observed at the foot, and on the summit-edge of a plain, ten miles from
the coast, many recent shells, mostly comminuted, but some perfect.
There were, also, many at the bottom of the great valley of the Maypu.
At San Antonio, shells are said to be collected and burnt for lime. At
the bottom of a great ravine (Quebrada Onda, on the road to Casa
Blanca), at the distance of several miles from the coast, I noticed a
considerable bed, composed exclusively of Mesodesma donaciforme, Desh.,
lying on a bed of muddy sand: this shell now lives associated together
in great numbers, on tidal-flats on the coast of Chile.
VALPARAISO.
During two successive years I carefully examined, part of the time in
company with Mr. Alison, into all the facts connected with the recent
elevation of this neighbourhood. In very many parts a beach of broken
shells, about fourteen or fifteen feet above high-water mark, may be
observed; and at this level the coast-rocks, where precipitous, are
corroded in a band. At one spot, Mr. Alison, by removing some birds’
dung, found at this same level barnacles adhering to the rocks. For
several miles southward of the bay, almost every flat little headland,
between the heights of 60 and 230 feet (measured by the barometer), is
smoothly coated by a thick mass of comminuted shells, of the same
species, and apparently in the same proportional numbers with those
existing in the adjoining sea. The Concholepas is much the most
abundant, and the best preserved shell; but I extracted perfectly
preserved specimens of the Fissurella biradiata, a Trochus and Balanus
(both well-known, but according to Mr. Sowerby yet unnamed) and parts
of the Mytilus Chiloensis. Most of these shells, as well as an
encrusting Nullipora, partially retain their colour; but they are
brittle, and often stained red from the underlying brecciated mass of
primary rocks; some are packed together, either in black or reddish
moulds; some lie loose on the bare rocky surfaces. The total number of
these shells is immense; they are less numerous, though still far from
rare, up a height of 1,000 feet above the sea. On the summit of a hill,
measured 557 feet, there was a small horizontal band of comminuted
shells, of which MANY consisted (and likewise from lesser heights) of
very young and small specimens of the still living Concholepas,
Trochus, Patellae, Crepidulae, and of Mytilus Magellanicus (?) (Mr.
Cuming informs me that he does not think this species identical with,
though closely resembling, the true M. Magellanicus of the southern and
eastern coast of South America; it lives abundantly on the coast of
Chile.): several of these shells were under a quarter of an inch in
their greatest diameter. My attention was called to this circumstance
by a native fisherman, whom I took to look at these shell-beds; and he
ridiculed the notion of such small shells having been brought up for
food; nor could some of the species have adhered when alive to other
larger shells. On another hill, some miles distant, and 648 feet high,
I found shells of the Concholepas and Trochus, perfect, though very
old, with fragments of Mytilus Chiloensis, all embedded in
reddish-brown mould: I also found these same species, with fragments of
an Echinus and of Balanus psittacus, on a hill 1,000 feet high. Above
this height, shells became very rare, though on a hill 1,300 feet high
(Measured by the barometer: the highest point in the range behind
Valparaiso I found to be 1,626 feet above the level of the sea.), I
collected the Concholepas, Trochus, Fissurella, and a Patella. At these
greater heights the shells are almost invariably embedded in mould, and
sometimes are exposed only by tearing up bushes. These shells obviously
had a very much more ancient appearance than those from the lesser
heights; the apices of the Trochi were often worn down; the little
holes made by burrowing animals were greatly enlarged; and the
Concholepas was often perforated quite through, owing to the inner
plates of shell having scaled off.
Many of these shells, as I have said, were packed in, and were quite
filled with, blackish or reddish-brown earth, resting on the granitic
detritus. I did not doubt until lately that this mould was of purely
terrestrial origin, when with a microscope examining some of it from
the inside of a Concholepas from the height of about one hundred feet,
I found that it was in considerable part composed of minute fragments
of the spines, mouth- bones, and shells of Echini, and of minute
fragments, of chiefly very young Patellae, Mytili, and other species. I
found similar microscopical fragments in earth filling up the central
orifices of some large Fissurellae. This earth when crushed emits a
sickly smell, precisely like that from garden-mould mixed with guano.
The earth accidentally preserved within the shells, from the greater
heights, has the same general appearance, but it is a little redder; it
emits the same smell when rubbed, but I was unable to detect with
certainty any marine remains in it. This earth resembles in general
appearance, as before remarked, that capping the rocks of Quiriquina in
the Bay of Concepcion, on which beds of sea-shells lay. I have, also,
shown that the black, peaty soil, in which the shells at the height of
350 feet at Chiloe were packed, contained many minute fragments of
marine animals. These facts appear to me interesting, as they show that
soils, which would naturally be considered of purely terrestrial
nature, may owe their origin in chief part to the sea.
Being well aware from what I have seen at Chiloe and in Tierra del
Fuego, that vast quantities of shells are carried, during successive
ages, far inland, where the inhabitants chiefly subsist on these
productions, I am bound to state that at greater heights than 557 feet,
where the number of very young and small shells proved that they had
not been carried up for food, the only evidence of the shells having
been naturally left by the sea, consists in their invariable and
uniform appearance of extreme antiquity—in the distance of some of the
places from the coast, in others being inaccessible from the nearest
part of the beach, and in the absence of fresh water for men to
drink—in the shells NOT LYING IN HEAPS,—and, lastly, in the close
similarity of the soil in which they are embedded, to that which lower
down can be unequivocally shown to be in great part formed from the
debris of the sea animals. (In the “Proceedings of the Geological
Society” volume 2 page 446, I have given a brief account of the
upraised shells on the coast of Chile, and have there stated that the
proofs of elevation are not satisfactory above the height of 230 feet.
I had at that time unfortunately overlooked a separate page written
during my second visit to Valparaiso, describing the shells now in my
possession from the 557 feet hill; I had not then unpacked my
collections, and had not reconsidered the obvious appearance of greater
antiquity of the shells from the greater heights, nor had I at that
time discovered the marine origin of the earth in which many of the
shells are packed. Considering these facts, I do not now feel a shadow
of doubt that the shells, at the height of 1,300 feet, have been
upraised by natural causes into their present position.)
With respect to the position in which the shells lie, I was repeatedly
struck here, at Concepcion, and at other places, with the frequency of
their occurrence on the summits and edges either of separate hills, or
of little flat headlands often terminating precipitously over the sea.
The several above-enumerated species of mollusca, which are found
strewed on the surface of the land from a few feet above the level of
the sea up to the height of 1,300 feet, all now live either on the
beach, or at only a few fathoms’ depth: Mr. Edmondston, in a letter to
Professor E. Forbes, states that in dredging in the Bay of Valparaiso,
he found the common species of Concholepas, Fissurella, Trochus,
Monoceros, Chitons, etc., living in abundance from the beach to a depth
of seven fathoms; and dead shells occurred only a few fathoms deeper.
The common Turritella cingulata was dredged up living at even from ten
to fifteen fathoms; but this is a species which I did not find here
amongst the upraised shells. Considering this fact of the species being
all littoral or sub-littoral, considering their occurrence at various
heights, their vast numbers, and their generally comminuted state,
there can be little doubt that they were left on successive beach-lines
during a gradual elevation of the land. The presence, however, of so
many whole and perfectly preserved shells appears at first a difficulty
on this view, considering that the coast is exposed to the full force
of an open ocean: but we may suppose, either that these shells were
thrown during gales on flat ledges of rock just above the level of
high-water mark, and that during the elevation of the land they are
never again touched by the waves, or, that during earthquakes, such as
those of 1822, 1835, and 1837, rocky reefs covered with marine-animals
were it one blow uplifted above the future reach of the sea. This
latter explanation is, perhaps, the most probable one with respect to
the beds at Concepcion entirely composed of the Mytilus Chiloensis, a
species which lives below the lowest tides; and likewise with respect
to the great beds occurring both north and south of Valparaiso, of the
Mesodesma donaciforme,—a shell which, as I am informed by Mr. Cuming,
inhabits sandbanks at the level of the lowest tides. But even in the
case of shells having the habits of this Mytilus and Mesodesma, beds of
them, wherever the sea gently throws up sand or mud, and thus protects
its own accumulations, might be upraised by the slowest movement, and
yet remain undisturbed by the waves of each new beach-line.
It is worthy of remark, that nowhere near Valparaiso above the height
of twenty feet, or rarely of fifty feet, I saw any lines of erosion on
the solid rocks, or any beds of pebbles; this, I believe, may be
accounted for by the disintegrating tendency of most of the rocks in
this neighbourhood. Nor is the land here modelled into terraces: Mr.
Alison, however, informs me, that on both sides of one narrow ravine,
at the height of 300 feet above the sea, he found a succession of
rather indistinct step-formed beaches, composed of broken shells, which
together covered a space of about eighty feet vertical.
I can add nothing to the accounts already published of the elevation of
the land at Valparaiso, which accompanied the earthquake of 1822 (Dr.
Meyen “Reise um Erde” Th. 1 s. 221, found in 1831 seaweed and other
bodies still adhering to some rocks which during the shock of 1822 were
lifted above the sea.): but I heard it confidently asserted, that a
sentinel on duty, immediately after the shock, saw a part of a fort,
which previously was not within the line of his vision, and this would
indicate that the uplifting was not horizontal: it would even appear
from some facts collected by Mr. Alison, that only the eastern half of
the bay was then elevated. Through the kindness of this same gentleman,
I am able to give an interesting account of the changes of level, which
have supervened here within historical periods: about the year 1680 a
long sea-wall (or Prefil) was built, of which only a few fragments now
remain; up to the year 1817, the sea often broke over it, and washed
the houses on the opposite side of the road (where the prison now
stands); and even in 1819, Mr. J. Martin remembers walking at the foot
of this wall, and being often obliged to climb over it to escape the
waves. There now stands (1834) on the seaward side of this wall, and
between it and the beach, in one part a single row of houses, and in
another part two rows with a street between them. This great extension
of the beach in so short a time cannot be attributed simply to the
accumulation of detritus; for a resident engineer measured for me the
height between the lowest part of the wall visible, and the present
beach-line at spring-tides, and the difference was eleven feet six
inches. The church of S. Augustin is believed to have been built in
1614, and there is a tradition that the sea formerly flowed very near
it; by levelling, its foundations were found to stand nineteen feet six
inches above the highest beach-line; so that we see in a period of 220
years, the elevation cannot have been as much as nineteen feet six
inches. From the facts given with respect to the sea-wall, and from the
testimony of the elder inhabitants, it appears certain that the change
in level began to be manifest about the year 1817. The only sudden
elevation of which there is any record occurred in 1822, and this seems
to have been less than three feet. Since that year, I was assured by
several competent observers, that part of an old wreck, which is firmly
embedded near the beach, has sensibly emerged; hence here, as at
Chiloe, a slow rise of the land appears to be now in progress. It seems
highly probable that the rocks which are corroded in a band at the
height of fourteen feet above the sea were acted on during the period,
when by tradition the base of S. Augustin church, now nineteen feet six
inches above the highest water-mark, was occasionally washed by the
waves.
VALPARAISO TO COQUIMBO.
For the first seventy-five miles north of Valparaiso I followed the
coast- road, and throughout this space I observed innumerable masses of
upraised shells. About Quintero there are immense accumulations (worked
for lime) of the Mesodesma donaciforme, packed in sandy earth; they
abound chiefly about fifteen feet above high-water, but shells are here
found, according to Mr. Miers, to a height of 500 feet, and at a
distance of three leagues from the coast (“Travels in Chile” volume 1
pages 395, 458. I received several similar accounts from the
inhabitants, and was assured that there are many shells on the plain of
Casa Blanca, between Valparaiso and Santiago, at the height of 800
feet.): I here noticed barnacles adhering to the rocks three or four
feet above the highest tides. In the neighbourhood of Plazilla and
Catapilco, at heights of between two hundred and three hundred feet,
the number of comminuted shells, with some perfect ones, especially of
the Mesodesma, packed in layers, was truly immense: the land at
Plazilla had evidently existed as a bay, with abrupt rocky masses
rising out of it, precisely like the islets in the broken bays now
indenting this coast. On both sides of the rivers Ligua, Longotomo,
Guachen, and Quilimari, there are plains of gravel about two hundred
feet in height, in many parts absolutely covered with shells. Close to
Conchalee, a gravel-plain is fronted by a lower and similar plain about
sixty feet in height, and this again is separated from the beach by a
wide tract of low land: the surfaces of all three plains or terraces
were strewed with vast numbers of the Concholepas, Mesodesma, an
existing Venus, and other still existing littoral shells. The two upper
terraces closely resemble in miniature the plains of Patagonia; and
like them are furrowed by dry, flat-bottomed, winding valleys.
Northward of this place I turned inward; and therefore found no more
shells: but the valleys of Chuapa, Illapel, and Limari, are bounded by
gravel-capped plains, often including a lower terrace within. These
plains send bay-like arms between and into the surrounding hills; and
they are continuously united with other extensive gravel-capped plains,
separating the coast mountain-ranges from the Cordillera.
COQUIMBO.
A narrow fringe-like plain, gently inclined towards the sea, here
extends for eleven miles along the coast, with arms stretching up
between the coast-mountains, and likewise up the valley of Coquimbo: at
its southern extremity it is directly connected with the plain of
Limari, out of which hills abruptly rise like islets, and other hills
project like headlands on a coast. The surface of the fringe-like plain
appears level, but differs insensibly in height, and greatly in
composition, in different parts.
At the mouth of the valley of Coquimbo, the surface consists wholly of
gravel, and stands from 300 to 350 feet above the level of the sea,
being about one hundred feet higher than in other parts. In these other
and lower parts the superficial beds consist of calcareous matter, and
rest on ancient tertiary deposits hereafter to be described. The
uppermost calcareous layer is cream-coloured, compact,
smooth-fractured, sub- stalactiform, and contains some sand, earthy
matter, and recent shells. It lies on, and sends wedge-like veins into,
a much more friable, calcareous, tuff-like variety; and both rest on a
mass about twenty feet in thickness, formed of fragments of recent
shells, with a few whole ones, and with small pebbles firmly cemented
together. (In many respects this upper hard, and the underlying more
friable, varieties, resemble the great superficial beds at King
George’s Sound in Australia, which I have described in my “Geological
Observations on Volcanic Islands.” There could be little doubt that the
upper layers there have been hardened by the action of rain on the
friable, calcareous matter, and that the whole mass has originated in
the decay of minutely comminuted sea-shells and corals.) This latter
rock is called by the inhabitants losa, and is used for building: in
many parts it is divided into strata, which dip at an angle of ten
degrees seaward, and appear as if they had originally been heaped in
successive layers (as may be seen on coral-reefs) on a steep beach.
This stone is remarkable from being in parts entirely formed of empty,
pellucid capsules or cells of calcareous matter, of the size of small
seeds: a series of specimens unequivocally showed that all these
capsules once contained minute rounded fragments of shells which have
since been gradually dissolved by water percolating through the mass.
(I have incidentally described this rock in the above work on Volcanic
Islands.)
The shells embedded in the calcareous beds forming the surface of this
fringe-like plain, at the height of from 200 to 250 feet above the sea,
consist of:—
1. Venus opaca. 2. Mulinia Byronensis. 3. Pecten purpuratus. 4.
Mesodesma donaciforme. 5. Turritella cingulata. 6. Monoceros costatum.
7. Concholepas Peruviana. 8. Trochus (common Valparaiso species). 9.
Calyptraea Byronensis.
Although these species are all recent, and are all found in the
neighbouring sea, yet I was particularly struck with the difference in
the proportional numbers of the several species, and of those now cast
up on the present beach. I found only one specimen of the Concholepas,
and the Pecten was very rare, though both these shells are now the
commonest kinds, with the exception, perhaps, of the Calyptraea
radians, of which I did not find one in the calcareous beds. I will not
pretend to determine how far this difference in the proportional
numbers depends on the age of the deposit, and how far on the
difference in nature between the present sandy beaches and the
calcareous bottom, on which the embedded shells must have lived.
(DIAGRAM 8.—SECTION OF PLAIN OF COQUIMBO.
A Section through Plain B-B and Ravine A.
Surface of plain 252 feet above sea.
A. Stratified sand, with recent shells in same proportions as on the
beach, half filling up a ravine.
B. Surface of plain, with scattered shells in nearly same proportions
as on the beach.
C. Upper calcareous bed, and D. Lower calcareous sandy bed (Losa), both
with recent shells, but not in same proportions as on the beach.
E. Upper ferrugino-sandy old tertiary stratum, and F. Lower old
tertiary stratum, both with all, or nearly all, extinct shells.)
On the bare surface of the calcareous plain, or in a thin covering of
sand, there were lying, at a height from 200 to 252 feet, many recent
shells, which had a much fresher appearance than the embedded ones:
fragments of the Concholepas, and of the common Mytilus, still
retaining a tinge of its colour, were numerous, and altogether there
was manifestly a closer approach in proportional numbers to those now
lying on the beach. In a mass of stratified, slightly agglutinated
sand, which in some places covers up the lower half of the seaward
escarpment of the plain, the included shells appeared to be in exactly
the same proportional numbers with those on the beach. On one side of a
steep-sided ravine, cutting through the plain behind Herradura Bay, I
observed a narrow strip of stratified sand, containing similar shells
in similar proportional numbers; a section of the ravine is represented
in Diagram 8, which serves also to show the general composition of the
plain. I mention this case of the ravine chiefly because without the
evidence of the marine shells in the sand, any one would have supposed
that it had been hollowed out by simple alluvial action.
The escarpment of the fringe-like plain, which stretches for eleven
miles along the coast, is in some parts fronted by two or three narrow,
step- formed terraces, one of which at Herradura Bay expands into a
small plain. Its surface was there formed of gravel, cemented together
by calcareous matter; and out of it I extracted the following recent
shells, which are in a more perfect condition than those from the upper
plain:—
1. Calyptraea radians. 2. Turritella cingulata. 3. Oliva Peruviana. 4.
Murex labiosus, var. 5. Nassa (identical with a living species). 6.
Solen Dombeiana. 7. Pecten purpuratus. 8. Venus Chilensis. 9.
Amphidesma rugulosum. The small irregular wrinkles of the posterior
part of this shell are rather stronger than in the recent specimens of
this species from Coquimbo. (G.B. Sowerby.) 10. Balanus (identical with
living species).
On the syenitic ridge, which forms the southern boundary of Herradura
Bay and Plain, I found the Concholepas and Turritella cingulata (mostly
in fragments), at the height of 242 feet above the sea. I could not
have told that these shells had not formerly been brought up by man, if
I had not found one very small mass of them cemented together in a
friable calcareous tuff. I mention this fact more particularly, because
I carefully looked, in many apparently favourable spots, at lesser
heights on the side of this ridge, and could not find even the smallest
fragment of a shell. This is only one instance out of many, proving
that the absence of sea-shells on the surface, though in many respects
inexplicable, is an argument of very little weight in opposition to
other evidence on the recent elevation of the land. The highest point
in this neighbourhood at which I found upraised shells of existing
species was on an inland calcareous plain, at the height of 252 feet
above the sea.
It would appear from Mr. Caldcleugh’s researches, that a rise has taken
place here within the last century and a half (“Proceedings of the
Geological Society” volume 2 page 446.); and as no sudden change of
level has been observed during the not very severe earthquakes, which
have occasionally occurred here, the rising has probably been slow,
like that now, or quite lately, in progress at Chiloe and at
Valparaiso: there are three well-known rocks, called the Pelicans,
which in 1710, according to Feuillee, were a fleur d’eau, but now are
said to stand twelve feet above low-water mark: the spring-tides rise
here only five feet. There is another rock, now nine feet above
high-water mark, which in the time of Frezier and Feuillee rose only
five or six feet out of water. Mr. Caldcleugh, I may add, also shows
(and I received similar accounts) that there has been a considerable
decrease in the soundings during the last twelve years in the Bays of
Coquimbo, Concepcion, Valparaiso, and Guasco; but as in these cases it
is nearly impossible to distinguish between the accumulation of
sediment and the upheavement of the bottom, I have not entered into any
details.
VALLEY OF COQUIMBO.
(FIGURE 9. EAST AND WEST SECTION THROUGH THE TERRACES AT COQUIMBO,
WHERE THEY DEBOUCH FROM THE VALLEY, AND FRONT THE SEA.
Vertical scale 1/10 of inch to 100 feet: horizontal scale much
contracted.
Height of terrace in feet from east (high) to west (low): Terrace F.
364 Terrace E. 302 Terrace D. shown dotted, height not given. Terrace
C. 120 Terrace B. 70 Terrace A. 25 sloping down to level of sea at Town
of Coquimbo.)
The narrow coast-plain sends, as before stated, an arm, or more
correctly a fringe, on both sides, but chiefly on the southern side,
several miles up the valley. These fringes are worn into steps or
terraces, which present a most remarkable appearance, and have been
compared (though not very correctly) by Captain Basil Hall, to the
parallel roads of Glen Roy in Scotland: their origin has been ably
discussed by Mr. Lyell. (“Principles of Geology” 1st edition volume 3
page 131.) The first section which I will give (Figure 9), is not drawn
across the valley, but in an east and west line at its mouth, where the
step-formed terraces debouch and present their very gently inclined
surfaces towards the Pacific.
The bottom plain (A) is about a mile in width, and rises quite
insensibly from the beach to a height of twenty-five feet at the foot
of the next plain; it is sandy, and abundantly strewed with shells.
Plain or terrace B is of small extent, and is almost concealed by the
houses of the town, as is likewise the escarpment of terrace C. On both
sides of a ravine, two miles south of the town, there are two little
terraces, one above the other, evidently corresponding with B and C;
and on them marine remains of the species already enumerated were
plentiful. Terrace E is very narrow, but quite distinct and level; a
little southward of the town there were traces of a terrace D
intermediate between E and C. Terrace F is part of the fringe-like
plain, which stretches for the eleven miles along the coast; it is here
composed of shingle, and is 100 feet higher than where composed of
calcareous matter. This greater height is obviously due to the quantity
of shingle, which at some former period has been brought down the great
valley of Coquimbo.
Considering the many shells strewed over the terraces A, B, and C, and
a few miles southward on the calcareous plain, which is continuously
united with the upper step-like plain F, there cannot, I apprehend, be
any doubt, that these six terraces have been formed by the action of
the sea; and that their five escarpments mark so many periods of
comparative rest in the elevatory movement, during which the sea wore
into the land. The elevation between these periods may have been sudden
and on AN AVERAGE not more than seventy-two feet each time, or it may
have been gradual and insensibly slow. From the shells on the three
lower terraces, and on the upper one, and I may add on the three
gravel-capped terraces at Conchalee, being all littoral and
sub-littoral species, and from the analogical facts given at
Valparaiso, and lastly from the evidence of a slow rising lately or
still in progress here, it appears to me far more probable that the
movement has been slow. The existence of these successive escarpments,
or old cliff- lines, is in another respect highly instructive, for they
show periods of comparative rest in the elevatory movement, and of
denudation, which would never even have been suspected from a close
examination of many miles of coast southward of Coquimbo.
(FIGURE 10. NORTH AND SOUTH SECTION ACROSS THE VALLEY OF COQUIMBO.
From north F (high) through E?, D, C, B, A (low), B?, C, D?, E, F
(high).
Vertical scale 1/10 of inch to 100 feet: horizontal scale much
contracted.
Terraces marked with ? do not occur on that side of the valley, and are
introduced only to make the diagram more intelligible. A river and
bottom- plain of valley C, E, and F, on the south side of valley, are
respectively, 197, 377, and 420 feet above the level of the sea.
AA. The bottom of the valley, believed to be 100 feet above the sea: it
is continuously united with the lowest plain A of Figure 9.
B. This terrace higher up the valley expands considerably; seaward it
is soon lost, its escarpment being united with that of C: it is not
developed at all on the south side of the valley.
C. This terrace, like the last, is considerably expanded higher up the
valley. These two terraces apparently correspond with B and C of Figure
9.
D is not well developed in the line of this section; but seaward it
expands into a plain: it is not present on the south side of the
valley; but it is met with, as stated under the former section, a
little south of the town.
E is well developed on the south side, but absent on the north side of
the valley: though not continuously united with E of Figure 9, it
apparently corresponds with it.
F. This is the surface-plain, and is continuously united with that
which stretches like a fringe along the coast. In ascending the valley
it gradually becomes narrower, and is at last, at the distance of about
ten miles from the sea, reduced to a row of flat-topped patches on the
sides of the mountains. None of the lower terraces extend so far up the
valley.)
We come now to the terraces on the opposite sides of the east and west
valley of Coquimbo: the section in Figure 10 is taken in a north and
south line across the valley at a point about three miles from the sea.
The valley measured from the edges of the escarpments of the upper
plain FF is about a mile in width; but from the bases of the bounding
mountains it is from three to four miles wide. The terraces marked with
an interrogative do not exist on that side of the valley, but are
introduced merely to render the diagram more intelligible.
These five terraces are formed of shingle and sand; three of them, as
marked by Captain B. Hall (namely, B, C, and F), are much more
conspicuous than the others. From the marine remains copiously strewed
at the mouth of the valley on the lower terraces, and southward of the
town on the upper one, they are, as before remarked, undoubtedly of
marine origin; but within the valley, and this fact well deserves
notice, at a distance of from only a mile and a half to three or four
miles from the sea, I could not find even a fragment of a shell.
ON THE INCLINATION OF THE TERRACES OF COQUIMBO, AND ON THE UPPER AND
BASAL EDGES OF THEIR ESCARPMENTS NOT BEING HORIZONTAL.
The surfaces of these terraces slope in a slight degree, as shown by
the sections in Figures 9 and 10 taken conjointly, both towards the
centre of the valley, and seawards towards its mouth. This double or
diagonal inclination, which is not the same in the several terraces,
is, as we shall immediately see, of simple explanation. There are,
however, some other points which at first appear by no means
obvious,—namely, first, that each terrace, taken in its whole breadth
from the summit-edge of one escarpment to the base of that above it,
and followed up the valley, is not horizontal; nor have the several
terraces, when followed up the valley, all the same inclination; thus I
found the terraces C, E, and F, measured at a point about two miles
from the mouth of the valley, stood severally between fifty-six to
seventy-seven feet higher than at the mouth. Again, if we look to any
one line of cliff or escarpment, neither its summit-edge nor its base
is horizontal. On the theory of the terraces having been formed during
a slow and equable rise of the land, with as many intervals of rest as
there are escarpments, it appears at first very surprising that
horizontal lines of some kind should not have been left on the land.
The direction of the diagonal inclination in the different terraces
being different,—in some being directed more towards the middle of the
valley, in others more towards its mouth,—naturally follows on the view
of each terrace, being an accumulation of successive beach-lines round
bays, which must have been of different forms and sizes when the land
stood at different levels: for if we look to the actual beach of a
narrow creek, its slope is directed towards the middle; whereas, in an
open bay, or slight concavity on a coast, the slope is towards the
mouth, that is, almost directly seaward; hence as a bay alters in form
and size, so will the direction of the inclination of its successive
beaches become changed.
(FIGURE 11. DIAGRAM OF A BAY IN A DISTRICT WHICH HAS BEGUN SLOWLY
RISING)
If it were possible to trace any one of the many beach-lines, composing
each sloping terrace, it would of course be horizontal; but the only
lines of demarcation are the summit and basal edges of the escarpments.
Now the summit-edge of one of these escarpments marks the furthest line
or point to which the sea has cut into a mass of gravel sloping
seaward; and as the sea will generally have greater power at the mouth
than at the protected head of the bay, so will the escarpment at the
mouth be cut deeper into the land, and its summit-edge be higher;
consequently it will not be horizontal. With respect to the basal or
lower edges of the escarpments, from picturing in one’s mind ancient
bays ENTIRELY surrounded at successive periods by cliff-formed shores,
one’s first impression is that they at least necessarily must be
horizontal, if the elevation has been horizontal. But here is a
fallacy: for after the sea has, during a cessation of the elevation,
worn cliffs all round the shores of a bay, when the movement
recommences, and especially if it recommences slowly, it might well
happen that, at the exposed mouth of the bay, the waves might continue
for some time wearing into the land, whilst in the protected and upper
parts successive beach-lines might be accumulating in a sloping surface
or terrace at the foot of the cliffs which had been lately reached:
hence, supposing the whole line of escarpment to be finally uplifted
above the reach of the sea, its basal line or foot near the mouth will
run at a lower level than in the upper and protected parts of the bay;
consequently this basal line will not be horizontal. And it has already
been shown that the summit-edges of each escarpment will generally be
higher near the mouth (from the seaward sloping land being there most
exposed and cut into) than near the head of the bay; therefore the
total height of the escarpments will be greatest near the mouth; and
further up the old bay or valley they will on both sides generally thin
out and die away: I have observed this thinning out of the successive
escarpment at other places besides Coquimbo; and for a long time I was
quite unable to understand its meaning. The rude diagram in Figure 11
will perhaps render what I mean more intelligible; it represents a bay
in a district which has begun slowly rising. Before the movement
commenced, it is supposed that the waves had been enabled to eat into
the land and form cliffs, as far up, but with gradually diminishing
power, as the points AA: after the movement had commenced and gone on
for a little time, the sea is supposed still to have retained the
power, at the exposed mouth of the bay, of cutting down and into the
land as it slowly emerged; but in the upper parts of the bay it is
supposed soon to have lost this power, owing to the more protected
situation and to the quantity of detritus brought down by the river;
consequently low land was there accumulated. As this low land was
formed during a slow elevatory movement, its surface will gently slope
upwards from the beach on all sides. Now, let us imagine the bay, not
to make the diagram more complicated, suddenly converted into a valley:
the basal line of the cliffs will of course be horizontal, as far as
the beach is now seen extending in the diagram; but in the upper part
of the valley, this line will be higher, the level of the district
having been raised whilst the low land was accumulating at the foot of
the inland cliffs. If, instead of the bay in the diagram being suddenly
converted into a valley, we suppose with much more probability it to be
upraised slowly, then the waves in the upper parts of the bay will
continue very gradually to fail to reach the cliffs, which are now in
the diagram represented as washed by the sea, and which, consequently,
will be left standing higher and higher above its level; whilst at the
still exposed mouth, it might well happen that the waves might be
enabled to cut deeper and deeper, both down and into the cliffs, as the
land slowly rose.
The greater or lesser destroying power of the waves at the mouths of
successive bays, comparatively with this same power in their upper and
protected parts, will vary as the bays become changed in form and size,
and therefore at different levels, at their mouths and heads, more or
less of the surfaces between the escarpments (that is, the accumulated
beach-lines or terraces) will be left undestroyed: from what has gone
before we can see that, according as the elevatory movements after each
cessation recommence more or less slowly, according to the amount of
detritus delivered by the river at the heads of the successive bays,
and according to the degree of protection afforded by their altered
forms, so will a greater or less extent of terrace be accumulated in
the upper part, to which there will be no surface at a corresponding
level at the mouth: hence we can perceive why no one terrace, taken in
its whole breadth and followed up the valley, is horizontal, though
each separate beach-line must have been so; and why the inclination of
the several terraces, both transversely, and longitudinally up the
valley, is not alike.
I have entered into this case in some detail, for I was long perplexed
(and others have felt the same difficulty) in understanding how, on the
idea of an equable elevation with the sea at intervals eating into the
land, it came that neither the terraces nor the upper nor lower edges
of the escarpments were horizontal. Along lines of coast, even of great
lengths, such as that of Patagonia, if they are nearly uniformly
exposed, the corroding power of the waves will be checked and conquered
by the elevatory movement, as often as it recommences, at about the
same period; and hence the terraces, or accumulated beach-lines, will
commence being formed at nearly the same levels: at each succeeding
period of rest, they will, also, be eaten into at nearly the same rate,
and consequently there will be a much closer coincidence in their
levels and inclinations, than in the terraces and escarpments formed
round bays with their different parts very differently exposed to the
action of the sea. It is only where the waves are enabled, after a long
lapse of time, slowly to corrode hard rocks, or to throw up, owing to
the supply of sediment being small and to the surface being steeply
inclined, a narrow beach or mound, that we can expect, as at Glen Roy
in Scotland (“Philosophical Transactions” 1839 page 39.), a distinct
line marking an old sea-level, and which will be strictly horizontal,
if the subsequent elevatory movements have been so: for in these cases
no discernible effects will be produced, except during the long
intervening periods of rest; whereas in the case of step-formed coasts,
such as those described in this and the preceding chapter, the terraces
themselves are accumulated during the slow elevatory process, the
accumulation commencing sooner in protected than in exposed situations,
and sooner where there is copious supply of detritus than where there
is little; on the other hand, the steps or escarpments are formed
during the stationary periods, and are more deeply cut down and into
the coast-land in exposed than in protected situations;—the cutting
action, moreover, being prolonged in the most exposed parts, both
during the beginning and ending, if slow, of the upward movement.
Although in the foregoing discussion I have assumed the elevation to
have been horizontal, it may be suspected, from the considerable
seaward slope of the terraces, both up the valley of S. Cruz and up
that of Coquimbo, that the rising has been greater inland than nearer
the coast. There is reason to believe (Mr. Place in the “Quarterly
Journal of Science” 1824 volume 17 page 42.), from the effects produced
on the water-course of a mill during the earthquake of 1822 in Chile,
that the upheaval one mile inland was nearly double, namely, between
five and seven feet, to what it was on the Pacific. We know, also, from
the admirable researches of M. Bravais, that in Scandinavia the ancient
sea-beaches gently slope from the interior mountain-ranges towards the
coast, and that they are not parallel one to the other (“Voyages de la
Comm. du Nord” etc. also “Comptes Rendus” October 1842.), showing that
the proportional difference in the amount of elevation on the coast and
in the interior, varied at different periods.
COQUIMBO TO GUASCO.
In this distance of ninety miles, I found in almost every part marine
shells up to a height of apparently from two hundred to three hundred
feet. The desert plain near Choros is thus covered; it is bounded by
the escarpment of a higher plain, consisting of pale-coloured, earthy,
calcareous stone, like that of Coquimbo, with the same recent shells
embedded in it. In the valley of Chaneral, a similar bed occurs in
which, differently from that of Coquimbo, I observed many shells of the
Concholepas: near Guasco the same calcareous bed is likewise met with.
In the valley of Guasco, the step-formed terraces of gravel are
displaced in a more striking manner than at any other point. I followed
the valley for thirty-seven miles (as reckoned by the inhabitants) from
the coast to Ballenar; in nearly the whole of this distance, five grand
terraces, running at corresponding heights on both sides of the broad
valley, are more conspicuous than the three best-developed ones at
Coquimbo. They give to the landscape the most singular and formal
aspect; and when the clouds hung low, hiding the neighbouring
mountains, the valley resembled in the most striking manner that of
Santa Cruz. The whole thickness of these terraces or plains seems
composed of gravel, rather firmly aggregated together, with occasional
parting seams of clay: the pebbles on the upper plain are often
whitewashed with an aluminous substance, as in Patagonia. Near the
coast I observed many sea-shells on the lower plains. At Freyrina
(twelve miles up the valley), there are six terraces beside the bottom-
surface of the valley: the two lower ones are here only from two
hundred to three hundred yards in width, but higher up the valley they
expand into plains; the third terrace is generally narrow; the fourth I
saw only in one place, but there it was distinct for the length of a
mile; the fifth is very broad; the sixth is the summit-plain, which
expands inland into a great basin. Not having a barometer with me, I
did not ascertain the height of these plains, but they appeared
considerably higher than those at Coquimbo. Their width varies much,
sometimes being very broad, and sometimes contracting into mere fringes
of separate flat-topped projections, and then quite disappearing: at
the one spot, where the fourth terrace was visible, the whole six
terraces were cut off for a short space by one single bold escarpment.
Near Ballenar (thirty-seven miles from the mouth of the river), the
valley between the summit-edges of the highest escarpments is several
miles in width, and the five terraces on both sides are broadly
developed: the highest cannot be less than six hundred feet above the
bed of the river, which itself must, I conceive, be some hundred feet
above the sea.
A north and south section across the valley in this part is represented
in Figure 12.
(FIGURE 12. NORTH AND SOUTH SECTION ACROSS THE VALLEY OF GUASCO, AND OF
A PLAIN NORTH OF IT.
From left (north, high) to right (south, high) through plains B and A
and the River of Guasco at the Town of Ballenar.)
On the northern side of the valley the summit-plain of gravel, A, has
two escarpments, one facing the valley, and the other a great
basin-like plain, B, which stretches for several leagues northward.
This narrow plain, A, with the double escarpment, evidently once formed
a spit or promontory of gravel, projecting into and dividing two great
bays, and subsequently was worn on both sides into steep cliffs.
Whether the several escarpments in this valley were formed during the
same stationary periods with those of Coquimbo, I will not pretend to
conjecture; but if so the intervening and subsequent elevatory
movements must have been here much more energetic, for these plains
certainly stand at a much higher level than do those of Coquimbo.
COPIAPO.
From Guasco to Copiapo, I followed the road near the foot of the
Cordillera, and therefore saw no upraised remains. At the mouth,
however, of the valley of Copiapo there is a plain, estimated by Meyen
(“Reise um die Erde” th. 1 s. 372 et seq.) between fifty and seventy
feet in height, of which the upper part consists chiefly of gravel,
abounding with recent shells, chiefly of the Concholepas, Venus
Dombeyi, and Calyptraea trochiformis. A little inland, on a plain
estimated by myself at nearly three hundred feet, the upper stratum was
formed of broken shells and sand cemented by white calcareous matter,
and abounding with embedded recent shells, of which the Mulinia
Byronensis and Pecten purpuratus were the most numerous. The lower
plain stretches for some miles southward, and for an unknown distance
northward, but not far up the valley; its seaward face, according to
Meyen, is worn into caves above the level of the present beach. The
valley of Copiapo is much less steeply inclined and less direct in its
course than any other valley which I saw in Chile; and its bottom does
not generally consist of gravel: there are no step-formed terraces in
it, except at one spot near the mouth of the great lateral valley of
the Despoblado where there are only two, one above the other: lower
down the valley, in one place I observed that the solid rock had been
cut into the shape of a beach, and was smoothed over with shingle.
Northward of Copiapo, in latitude 26 degrees S., the old voyager Wafer
found immense numbers of sea-shells some miles from the coast.
(Burnett’s “Collection of Voyages” volume 4 page 193.) At Cobija
(latitude 22 degrees 34′) M. d’Orbigny observed beds of gravel and
broken shells, containing ten species of recent shells; he also found,
on projecting points of porphyry, at a height of 300 feet, shells of
Concholepas, Chiton, Calyptraea, Fissurella, and Patella, still
attached to the spots on which they had lived. M. d’Orbigny argues from
this fact, that the elevation must have been great and sudden (“Voyage,
Part Geolog.” page 94. M. d’Orbigny (page 98), in summing up, says:
“S’il est certain (as he believes) que tous les terrains en pente,
compris entre la mer et les montagnes sont l’ancien rivage de la mer,
on doit supposer, pour l’ensemble, un exhaussement que ce ne serait pas
moindre de deux cent metres; il faudrait supposer encore que ce
soulevement n’a point ete graduel;...mais qu’il resulterait d’une seule
et meme cause fortuite,” etc. Now, on this view, when the sea was
forming the beach at the foot of the mountains, many shells of
Concholepas, Chiton, Calyptraea, Fissurella, and Patella (which are
known to live close to the beach), were attached to rocks at a depth of
300 feet, and at a depth of 600 feet several of these same shells were
accumulating in great numbers in horizontal beds. From what I have
myself seen in dredging, I believe this to be improbable in the highest
degree, if not impossible; and I think everyone who has read Professor
E. Forbes’s excellent researches on the subject, will without
hesitation agree in this conclusion.): to me it appears far more
probable that the movement was gradual, with small starts as during the
earthquakes of 1822 and 1835, by which whole beds of shells attached to
the rocks were lifted above the subsequent reach of the waves. M.
d’Orbigny also found rolled pebbles extending up the mountain to a
height of at least six hundred feet. At Iquique (latitude 20 degrees
12′ S.), in a great accumulation of sand, at a height estimated between
one hundred and fifty and two hundred feet, I observed many large
sea-shells which I thought could not have been blown up by the wind to
that height. Mr. J.H. Blake has lately described these shells: he
states that “inland toward the mountains they form a compact uniform
bed, scarcely a trace of the original shells being discernible; but as
we approach the shore, the forms become gradually more distinct till we
meet with the living shells on the coast.” (“Silliman’s American
Journal of Science” volume 44 page 2.) This interesting observation,
showing by the gradual decay of the shells how slowly and gradually the
coast must have been uplifted, we shall presently see fully confirmed
at Lima. At Arica (latitude 18 degrees 28′), M. d’Orbigny found a great
range of sand-dunes, fourteen leagues in length, stretching towards
Tacna, including recent shells and bones of Cetacea, and reaching up to
a height of 300 feet above the sea. (“Voyage” etc. page 101.)
Lieutenant Freyer has given some more precise facts: he states (In a
letter to Mr. Lyell “Geological Proceedings” volume 2 page 179.) that
the Morro of Arica is about four hundred feet high; it is worn into
obscure terraces, on the bare rock of which he found Balini and
Milleporae adhering. At the height of between twenty and thirty feet
the shells and corals were in a quite fresh state, but at fifty feet
they were much abraded; there were, however, traces of organic remains
at greater heights. On the road from Tacna to Arequipa, between
Loquimbo and Moquegua, Mr. M. Hamilton found numerous recent sea shells
in sand, at a considerable distance from the sea. (“Edinburgh New
Philosophical Journal” volume 30 page 155.)
LIMA.
Northward of Arica, I know nothing of the coast for about a space of
five degrees of latitude; but near Callao, the port of Lima, there is
abundant and very curious evidence of the elevation of the land. The
island of San Lorenzo is upwards of one thousand feet high; the basset
edges of the strata composing the lower part are worn into three
obscure, narrow, sloping steps or ledges, which can be seen only when
standing on them: they probably resemble those described by Lieutenant
Freyer at Arica. The surface of the lower ledge, which extends from a
low cliff overhanging the sea to the foot of the next upper escarpment,
is covered by an enormous accumulation of recent shells. (M. Chevalier,
in the “Voyage of the ‘Bonite’” observed these shells; but his
specimens were lost.—“L’Institut” 1838 page 151.) The bed is level, and
in some parts more than two feet in thickness; I traced it over a space
of one mile in length, and heard of it in other places: the uppermost
part is eighty-five feet by the barometer above high-water mark. The
shells are packed together, but not stratified: they are mingled with
earth and stones, and are generally covered by a few inches of
detritus; they rest on a mass of nearly angular fragments of the
underlying sandstone, sometimes cemented together by common salt. I
collected eighteen species of shells of all ages and sizes. Several of
the univalves had evidently long lain dead at the bottom of the sea,
for their INSIDES were incrusted with Balani and Serpulae. All,
according to Mr. G.B. Sowerby, are recent species: they consist of:—
1. Mytilus Magellanicus: same as that found at Valparaiso, and there
stated to be probably distinct from the true M. Magellanicus of the
east coast.
2. Venus costellata, Sowerby “Zoological Proceedings.”
3. Pecten purpuratus, Lam.
4. Chama, probably echinulata, Brod.
5. Calyptraea Byronensis, Gray.
6. Calyptraea radians (Trochus, Lam.)
7. Fissurella affinis, Gray.
8. Fissurella biradiata, Trembly.
9. Purpura chocolatta, Duclos.
10. Purpura Peruviana, Gray.
11. Purpura labiata, Gray.
12. Purpura buxea (Murex, Brod.).
13. Concholepas Peruviana.
14. Nassa, related to reticulata.
15. Triton rudis, Brod.
16. Trochus, not yet described, but well-known and very common.
17 and 18. Balanus, two species, both common on the coast.
These upraised shells appear to be nearly in the same proportional
numbers- -with the exception of the Crepidulae being more numerous—with
those on the existing beach. The state of preservation of the different
species differed much; but most of them were much corroded, brittle,
and bleached: the upper and lower surfaces of the Concholepas had
generally quite scaled off: some of the Trochi and Fissurellae still
partially retain their colours. It is remarkable that these shells,
taken all together, have fully as ancient an appearance, although the
extremely arid climate appears highly favourable for their
preservation, as those from 1,300 feet at Valparaiso, and certainly a
more ancient appearance than those from five to six hundred feet from
Valparaiso and Concepcion; at which places I have seen grass and other
vegetables actually growing out of the shells. Many of the univalves
here at San Lorenzo were filled with, and united together by, pure
salt, probably left by the evaporation of the sea-spray, as the land
slowly emerged. (The underlying sandstone contains true layers of salt;
so that the salt may possibly have come from the beds in the higher
parts of the island; but I think more probably from the sea-spray. It
is generally asserted that rain never falls on the coast of Peru; but
this is not quite accurate; for, on several days, during our visit, the
so-called Peruvian dew fell in sufficient quantity to make the streets
muddy, and it would certainly have washed so deliquescent a substance
as salt into the soil. I state this because M. d’Orbigny, in discussing
an analogous subject, supposes that I had forgotten that it never rains
on this whole line of coast. See Ulloa’s “Voyage” volume 2 English
Translation page 67 for an account of the muddy streets of Lima, and on
the continuance of the mists during the whole winter. Rain, also, falls
at rare intervals even in the driest districts, as, for instance,
during forty days, in 1726, at Chocope (7 degrees 46′); this rain
entirely ruined (“Ulloa” etc. page 18) the mud houses of the
inhabitants.) On the highest parts of the ledge, small fragments of the
shells were mingled with, and evidently in process of reduction into, a
yellowish-white, soft, calcareous powder, tasting strongly of salt, and
in some places as fine as prepared medicinal chalk.
FOSSIL-REMAINS OF HUMAN ART.
In the midst of these shells on San Lorenzo, I found light corallines,
the horny ovule-cases of Mollusca, roots of seaweed (Mr. Smith of
Jordan Hill found pieces of seaweed in an upraised pleistocene deposit
in Scotland. See his admirable Paper in the “Edinburgh New
Philosophical Journal” volume 25 page 384.), bones of birds, the heads
of Indian corn and other vegetable matter, a piece of woven rushes, and
another of nearly decayed COTTON string. I extracted these remains by
digging a hole, on a level spot; and they had all indisputably been
embedded with the shells. I compared the plaited rush, the COTTON
string, and Indian corn, at the house of an antiquary, with similar
objects, taken from the Huacas or burial-grounds of the ancient
Peruvians, and they were undistinguishable; it should be observed that
the Peruvians used string only of cotton. The small quantity of sand or
gravel with the shells, the absence of large stones, the width and
thickness of the bed, and the time requisite for a ledge to be cut into
the sandstone, all show that these remains were not thrown high up by
an earthquake-wave: on the other hand, these facts, together with the
number of dead shells, and of floating objects, both marine and
terrestrial, both natural and human, render it almost certain that they
were accumulated on a true beach, since upraised eighty-five feet, and
upraised this much since INDIAN MAN INHABITED PERU. The elevation may
have been, either by several small sudden starts, or quite gradual; in
this latter case the unrolled shells having been thrown up during gales
beyond the reach of the waves which afterwards broke on the slowly
emerging land. I have made these remarks, chiefly because I was at
first surprised at the complete difference in nature, between this
broad, smooth, upraised bed of shells, and the present shingle-beach at
the foot of the low sandstone-cliffs; but a beach formed, when the sea
is cutting into the land, as is shown now to be the case by the low
bare sandstone-cliffs, ought not to be compared with a beach
accumulated on a gently inclined rocky surface, at a period when the
sea (probably owing to the elevatory movement in process) was not able
to eat into the land. With respect to the mass of nearly angular, salt-
cemented fragments of sandstone, which lie under the shells, and which
are so unlike the materials of an ordinary sea-beach; I think it
probable after having seen the remarkable effects of the earthquake of
1835 (I have described this in my “Journal of Researches” page 303 2nd
edition.), in absolutely shattering as if by gunpowder the SURFACE of
the primary rocks near Concepcion, that a smooth bare surface of stone
was left by the sea covered by the shelly mass, and that afterwards
when upraised, it was superficially shattered by the severe shocks so
often experienced here.
The very low land surrounding the town of Callao, is to the south
joined by an obscure escarpment to a higher plain (south of Bella
Vista), which stretches along the coast for a length of about eight
miles. This plain appears to the eye quite level; but the sea-cliffs
show that its height varies (as far as I could estimate) from seventy
to one hundred and twenty feet. It is composed of thin, sometimes
waving, beds of clay, often of bright red and yellow colours, of layers
of impure sand, and in one part with a great stratified mass of
granitic pebbles. These beds are capped by a remarkable mass, varying
from two to six feet in thickness, of reddish loam or mud, containing
many scattered and broken fragments of recent marine shells, sometimes
though rarely single large round pebble, more frequently short
irregular layers of fine gravel, and very many pieces of red coarse
earthenware, which from their curvatures must once have formed parts of
large vessels. The earthenware is of Indian manufacture; and I found
exactly similar pieces accidentally included within the bricks, of
which the neighbouring ancient Peruvian burial-mounds are built. These
fragments abounded in such numbers in certain spots, that it appeared
as if waggon-loads of earthenware had been smashed to pieces. The
broken sea- shells and pottery are strewed both on the surface, and
throughout the whole thickness of this upper loamy mass. I found them
wherever I examined the cliffs, for a space of between two and three
miles, and for half a mile inland; and there can be little doubt that
this same bed extends with a smooth surface several miles further over
the entire plain. Besides the little included irregular layers of small
pebbles, there are occasionally very obscure traces of stratification.
At one of the highest parts of the cliff, estimated 120 feet above the
sea, where a little ravine came down, there were two sections, at right
angles to each other, of the floor of a shed or building. In both
sections or faces, two rows, one over the other, of large round stones
could be distinctly seen; they were packed close together on an
artificial layer of sand two inches thick, which had been placed on the
natural clay-beds; the round stones were covered by three feet in
thickness of the loam with broken sea-shells and pottery. Hence, before
this widely spread-out bed of loam was deposited, it is certain that
the plain was inhabited; and it is probable, from the broken vessels
being so much more abundant in certain spots than in others, and from
the underlying clay being fitted for their manufacture, that the kilns
stood here.
The smoothness and wide extent of the plain, the bulk of matter
deposited, and the obscure traces of stratification seem to indicate
that the loam was deposited under water; on the other hand, the
presence of sea-shells, their broken state, the pebbles of various
sizes, and the artificial floor of round stones, almost prove that it
must have originated in a rush of water from the sea over the land. The
height of the plain, namely, 120 feet, renders it improbable that an
earthquake-wave, vast as some have here been, could have broken over
the surface at its present level; but when the land stood eighty-five
feet lower, at the period when the shells were thrown up on the ledge
at S. Lorenzo, and when as we know man inhabited this district, such an
event might well have occurred; and if we may further suppose, that the
plain was at that time converted into a temporary lake, as actually
occurred, during the earthquakes of 1713 and 1746, in the case of the
low land round Callao owing to its being encircled by a high
shingle-beach, all the appearances above described will be perfectly
explained. I must add, that at a lower level near the point where the
present low land round Callao joins the higher plain, there are
appearances of two distinct deposits both apparently formed by
debacles: in the upper one, a horse’s tooth and a dog’s jaw were
embedded; so that both must have been formed after the settlement of
the Spaniards: according to Acosta, the earthquake-wave of 1586 rose
eighty-four feet.
The inhabitants of Callao do not believe, as far as I could ascertain,
that any change in level is now in progress. The great fragments of
brickwork, which it is asserted can be seen at the bottom of the sea,
and which have been adduced as a proof of a late subsidence, are, as I
am informed by Mr. Gill, a resident engineer, loose fragments; this is
probable, for I found on the beach, and not near the remains of any
building, masses of brickwork, three and four feet square, which had
been washed into their present places, and smoothed over with shingle
during the earthquake of 1746. The spit of land, on which the ruins of
OLD Callao stand, is so extremely low and narrow, that it is improbable
in the highest degree that a town should have been founded on it in its
present state; and I have lately heard that M. Tschudi has come to the
conclusion, from a comparison of old with modern charts, that the coast
both south and north of Callao has subsided. (I am indebted for this
fact to Dr. E. Dieffenbach. I may add that there is a tradition, that
the islands of San Lorenzo and Fronton were once joined, and that the
channel between San Lorenzo and the mainland, now above two miles in
width, was so narrow that cattle used to swim over.) I have shown that
the island of San Lorenzo has been upraised eighty-five feet since the
Peruvians inhabited this country; and whatever may have been the amount
of recent subsidence, by so much more must the elevation have exceeded
the eighty-five feet. In several places in this neighbourhood, marks of
sea-action have been observed: Ulloa gives a detailed account of such
appearances at a point five leagues northward of Callao: Mr. Cruikshank
found near Lima successive lines of sea-cliffs, with rounded blocks at
their bases, at a height of 700 feet above the present level of the
sea. (“Observaciones sobre el Clima del Lima” par Dr. H. Unanue page
4.—Ulloa’s “Voyage” volume 2 English Translation page 97.—For Mr.
Cruikshank’s observations, see Mr. Lyell’s “Principles of Geology” 1st
edition volume 3 page 130.) ON THE DECAY OF UPRAISED SEA-SHELLS.
I have stated that many of the shells on the lower inclined ledge or
terrace of San Lorenzo are corroded in a peculiar manner, and that they
have a much more ancient appearance than the same species at
considerably greater heights on the coast of Chile. I have, also,
stated that these shells in the upper part of the ledge, at the height
of eighty-five feet above the sea, are falling, and in some parts are
quite changed into a fine, soft, saline, calcareous powder. The finest
part of this powder has been analysed for me, at the request of Sir H.
De la Beche, by the kindness of Mr. Trenham Reeks of the Museum of
Economic Geology; it consists of carbonate of lime in abundance, of
sulphate and muriate of lime, and of muriate and sulphate of soda. The
carbonate of lime is obviously derived from the shells; and common salt
is so abundant in parts of the bed, that, as before remarked, the
univalves are often filled with it. The sulphate of lime may have been
derived, as has probably the common salt, from the evaporation of the
sea-spray, during the emergence of the land; for sulphate of lime is
now copiously deposited from the spray on the shores of Ascension. (See
“Volcanic Islands” etc. by the Author.) The other saline bodies may
perhaps have been partially thus derived, but chiefly, as I conclude
from the following facts, through a different means.
On most parts of the second ledge or old sea-beach, at a height of 170
feet, there is a layer of white powder of variable thickness, as much
in some parts as two inches, lying on the angular, salt-cemented
fragments of sandstone and under about four inches of earth, which
powder, from its close resemblance in nature to the upper and most
decayed parts of the shelly mass, I can hardly doubt originally existed
as a bed of shells, now much collapsed and quite disintegrated. I could
not discover with the microscope a trace of organic structure in it;
but its chemical constituents, according to Mr. Reeks, are the same as
in the powder extracted from amongst the decaying shells on the lower
ledge, with the marked exception that the carbonate of lime is present
in only very small quantity. On the third and highest ledge, I observed
some of this powder in a similar position, and likewise occasionally in
small patches at considerably greater heights near the summit of the
island. At Iquique, where the whole face of the country is covered by a
highly saliferous alluvium, and where the climate is extremely dry, we
have seen that, according to Mr. Blake, the shells which are perfect
near the beach become, in ascending, gradually less and less perfect,
until scarcely a trace of their original structure can be discovered.
It is known that carbonate of lime and common salt left in a mass
together, and slightly moistened, partially decompose each other (I am
informed by Dr. Kane, through Mr. Reeks, that a manufactory was
established on this principle in France, but failed from the small
quantity of carbonate of soda produced. Sprengel “Gardeners’ Chronicle”
1845 page 157, states, that salt and carbonate of lime are liable to
mutual decomposition in the soil. Sir H. De la Beche informs me, that
calcareous rocks washed by the spray of the sea, are often corroded in
a peculiar manner; see also on this latter subject “Gardeners’
Chronicle” page 675 1844.): now we have at San Lorenzo and at Iquique,
in the shells and salt packed together, and occasionally moistened by
the so- called Peruvian dew, the proper elements for this action. We
can thus understand the peculiar corroded appearance of the shells on
San Lorenzo, and the great decrease of quantity in the carbonate of
lime in the powder on the upper ledge. There is, however, a great
difficulty on this view, for the resultant salts should be carbonate of
soda and muriate of lime; the latter is present, but not the carbonate
of soda. Hence I am led to the perhaps unauthorised conjecture (which I
shall hereafter have to refer to) that the carbonate of soda, by some
unexplained means, becomes converted into a sulphate.
If the above remarks be just, we are led to the very unexpected
conclusion, that a dry climate, by leaving the salt from the sea-spray
undissolved, is much less favourable to the preservation of upraised
shells than a humid climate. However this may be, it is interesting to
know the manner in which masses of shells, gradually upraised above the
sea-level, decay and finally disappear.
A SUMMARY ON THE RECENT ELEVATION OF THE WEST COAST OF SOUTH AMERICA.
We have seen that upraised marine remains occur at intervals, and in
some parts almost continuously, from latitude 45 degrees 35′ to 12
degrees S., along the shores of the Pacific. This is a distance, in a
north and south line, of 2,075 geographical miles. From Byron’s
observations, the elevation has no doubt extended sixty miles further
south; and from the similarity in the form of the country near Lima, it
has probably extended many leagues further north. (I may take this
opportunity of stating that in a MS. in the Geological Society by Mr.
Weaver, it is stated that beds of oysters and other recent shells are
found thirty feet above the level of the sea, in many parts of Tampico,
in the Gulf of Mexico.) Along this great line of coast, besides the
organic remains, there are in very many parts, marks of erosion, caves,
ancient beaches, sand-dunes, and successive terraces of gravel, all
above the present level of the sea. From the steepness of the land on
this side of the continent, shells have rarely been found at greater
distances inland than from two to three leagues; but the marks of
sea-action are evident farther from the coast; for instance, in the
valley of Guasco, at a distance of between thirty and forty miles.
Judging from the upraised shells alone, the elevation in Chiloe has
been 350 feet, at Concepcion certainly 625 feet; and by estimation
1,000 feet; at Valparaiso 1,300 feet; at Coquimbo 252 feet; northward
of this place, sea-shells have not, I believe, been found above 300
feet; and at Lima they were falling into decay (hastened probably by
the salt) at 85 feet. Not only has this amount of elevation taken place
within the period of existing Mollusca and Cirripedes; but their
proportional numbers in the neighbouring sea have in most cases
remained the same. Near Lima, however, a small change in this respect
between the living and the upraised was observed: at Coquimbo this was
more evident, all the shells being existing species, but with those
embedded in the uppermost calcareous plain not approximating so closely
in proportional numbers, as do those that lie loose on its surface at
the height of 252 feet, and still less closely than those which are
strewed on the lower plains, which latter are identical in proportional
numbers with those now cast up on the beach. From this circumstance,
and from not finding, upon careful examination, near Coquimbo any
shells at a greater height than 252 feet, I believe that the recent
elevation there has been much less than at Valparaiso, where it has
been 1,300 feet, and I may add, than at Concepcion. This considerable
inequality in the amount of elevation at Coquimbo and Valparaiso,
places only 200 miles apart, is not improbable, considering, first, the
difference in the force and number of the shocks now yearly affecting
different parts of this coast; and, secondly, the fact of single areas,
such as that of the province of Concepcion, having been uplifted very
unequally during the same earthquake. It would, in most cases, be very
hazardous to infer an inequality of elevation, from shells being found
on the surface or in superficial beds at different heights; for we do
not know on what their rate of decay depends; and at Coquimbo one
instance out of many has been given, of a promontory, which, from the
occurrence of one very small collection of lime-cemented shells, has
indisputably been elevated 242 feet, and yet on which, not even a
fragment of shell could be found on careful examination between this
height and the beach, although many sites appeared very favourable for
the preservation of organic remains: the absence, also, of shells on
the gravel-terraces a short distance up the valley of Coquimbo, though
abundant on the corresponding terraces at its mouth, should be borne in
mind.
There are other epochs, besides that of the existence of recent
Mollusca, by which to judge of the changes of level on this coast. At
Lima, as we have just seen, the elevation has been at least eighty-five
feet, within the Indo-human period; and since the arrival of the
Spaniards in 1530, there has apparently been a sinking of the surface.
At Valparaiso, in the course of 220 years, the rise must have been less
than nineteen feet; but it has been as much as from ten to eleven feet
in the seventeen years subsequently to 1817, and of this rise only a
part can be attributed to the earthquake of 1822, the remainder having
been insensible and apparently still, in 1834, in progress. At Chiloe
the elevation has been gradual, and about four feet during four years.
At Coquimbo, also, it has been gradual, and in the course of 150 years
has amounted to several feet. The sudden small upheavals, accompanied
by earthquakes, as in 1822 at Valparaiso, in 1835 at Concepcion, and in
1837 in the Chonos Archipelago, are familiar to most geologists, but
the gradual rising of the coast of Chile has been hardly noticed; it
is, however, very important, as connecting together these two orders of
events.
The rise of Lima, having been eighty-five feet within the period of
man, is the more surprising if we refer to the eastern coast of the
continent, for at Port S. Julian, in Patagonia, there is good evidence
(as we shall hereafter see) that when the land stood ninety feet lower,
the Macrauchenia, a mammiferous beast, was alive; and at Bahia Blanca,
when it stood only a few feet lower than it now does, many gigantic
quadrupeds ranged over the adjoining country. But the coast of
Patagonia is some way distant from the Cordillera, and the movement at
Bahia Blanca is perhaps noways connected with this great range, but
rather with the tertiary volcanic rocks of Banda Oriental, and
therefore the elevation at these places may have been infinitely slower
than on the coast of Peru. All such speculations, however, must be
vague, for as we know with certainty that the elevation of the whole
coast of Patagonia has been interrupted by many and long pauses, who
will pretend to say that, in such cases, many and long periods of
subsidence may not also have been intercalated?
In many parts of the coast of Chile and Peru there are marks of the
action of the sea at successive heights on the land, showing that the
elevation has been interrupted by periods of comparative rest in the
upward movement, and of denudation in the action of the sea. These are
plainest at Chiloe, where, in a height of about five hundred feet,
there are three escarpments,—at Coquimbo, where in a height of 364
feet, there are five,— at Guasco, where there are six, of which five
may perhaps correspond with those at Coquimbo, but if so, the
subsequent and intervening elevatory movements have been here much more
energetic,—at Lima, where, in a height of about 250 feet there are
three terraces, and others, as it is asserted, at considerably greater
heights. The almost entire absence of ancient marks of sea-action at
defined levels along considerable spaces of coast, as near Valparaiso
and Concepcion, is highly instructive, for as it is improbable that the
elevation at these places alone should have been continuous, we must
attribute the absence of such marks to the nature and form of the
coast-rocks. Seeing over how many hundred miles of the coast of
Patagonia, and on how many places on the shores of the Pacific, the
elevatory process has been interrupted by periods of comparative rest,
we may conclude, conjointly with the evidence drawn from other quarters
of the world, that the elevation of the land is generally an
intermittent action. From the quantity of matter removed in the
formation of the escarpments, especially of those of Patagonia, it
appears that the periods of rest in the movement, and of denudation of
the land, have generally been very long. In Patagonia, we have seen
that the elevation has been equable, and the periods of denudation
synchronous over very wide spaces of coast; on the shores of the
Pacific, owing to the terraces chiefly occurring in the valleys, we
have not equal means of judging on this point; and the very different
heights of the upraised shells at Coquimbo, Valparaiso, and Concepcion
seem directly opposed to such a conclusion.
Whether on this side of the continent the elevation, between the
periods of comparative rest when the escarpments were formed, has been
by small sudden starts, such as those accompanying recent earthquakes,
or, as is most probable, by such starts conjointly with a gradual
upward movement, or by great and sudden upheavals, I have no direct
evidence. But as on the eastern coast, I was led to think, from the
analogy of the last hundred feet of elevation in La Plata, and from the
nearly equal size of the pebbles over the entire width of the terraces,
and from the upraised shells being all littoral species, that the
elevation had been gradual; so do I on this western coast, from the
analogy of the movements now in progress, and from the vast numbers of
shells now living exclusively on or close to the beach, which are
strewed over the whole surface of the land up to very considerable
heights, conclude, that the movement here also has been slow and
gradual, aided probably by small occasional starts. We know at least
that at Coquimbo, where five escarpments occur in a height of 364 feet,
the successive elevations, if they have been sudden, cannot have been
very great. It has, I think, been shown that the occasional
preservation of shells, unrolled and unbroken, is not improbable even
during a quite gradual rising of the land; and their preservation, if
the movement has been aided by small starts, is quite conformable with
what actually takes place during recent earthquakes.
Judging from the present action of the sea, along the shores of the
Pacific, on the deposits of its own accumulation, the present time
seems in most places to be one of comparative rest in the elevatory
movement, and of denudation of the land. Undoubtedly this is the case
along the whole great length of Patagonia. At Chiloe, however, we have
seen that a narrow sloping fringe, covered with vegetation, separates
the present sea-beach from a line of low cliffs, which the waves lately
reached; here, then, the land is gaining in breadth and height, and the
present period is not one of rest in the elevation and of contingent
denudation; but if the rising be not prolonged at a quick rate, there
is every probability that the sea will soon regain its former
horizontal limits. I observed similar low sloping fringes on several
parts of the coast, both northward of Valparaiso and near Coquimbo; but
at this latter place, from the change in form which the coast has
undergone since the old escarpments were worn, it may be doubted
whether the sea, acting for any length of time at its present level,
would eat into the land; for it now rather tends to throw up great
masses of sand. It is from facts such as these that I have generally
used the term COMPARATIVE rest, as applied to the elevation of the
land; the rest or cessation in the movement being comparative both with
what has preceded it and followed it, and with the sea’s power of
corrosion at each spot and at each level. Near Lima, the cliff-formed
shores of San Lorenzo, and on the mainland south of Callao, show that
the sea is gaining on the land; and as we have here some evidence that
its surface has lately subsided or is still sinking, the periods of
comparative rest in the elevation and of contingent denudation, may
probably in many cases include periods of subsidence. It is only, as
was shown in detail when discussing the terraces of Coquimbo, when the
sea with difficulty and after a long lapse of time has either corroded
a narrow ledge into solid rock, or has heaped up on a steep surface a
NARROW mound of detritus, that we can confidently assert that the land
at that level and at that period long remained absolutely stationary.
In the case of terraces formed of gravel or sand, although the
elevation may have been strictly horizontal, it may well happen that no
one level beach-line may be traceable, and that neither the terraces
themselves nor the summit nor basal edges of their escarpments may be
horizontal.
Finally, comparing the extent of the elevated area, as deduced from the
upraised recent organic remains, on the two sides of the continent, we
have seen that on the Atlantic, shells have been found at intervals
from Eastern Tierra del Fuego for 1,180 miles northward, and on the
Pacific for a space of 2,075 miles. For a length of 775 miles, they
occur in the same latitudes on both sides of the continent. Without
taking this circumstance into consideration, it is probable from the
reasons assigned in the last chapter, that the entire breadth of the
continent in Central Patagonia has been uplifted in mass; but from
other reasons there given, it would be hazardous to extend this
conclusion to La Plata. From the continent being narrow in the
southern-most parts of Patagonia, and from the shells found at the
Inner Narrows of the Strait of Magellan, and likewise far up the valley
of the Santa Cruz, it is probable that the southern part of the western
coast, which was not visited by me, has been elevated within the period
of recent Mollusca: if so, the shores of the Pacific have been
continuously, recently, and in a geological sense synchronously
upraised, from Lima for a length of 2,480 nautical miles southward,—a
distance equal to that from the Red Sea to the North Cape of
Scandinavia!
CHAPTER III.
ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL DEPOSITS.
Basin-like plains of Chile; their drainage, their marine origin.—Marks
of sea-action on the eastern flanks of the Cordillera.—Sloping
terrace-like fringes of stratified shingle within the valleys of the
Cordillera; their marine origin.—Boulders in the valley of
Cachapual.—Horizontal elevation of the Cordillera.—Formation of
valleys.—Boulders moved by earthquake-waves.—Saline superficial
deposits.—Bed of nitrate of soda at Iquique.—Saline
incrustations.—Salt-lakes of La Plata and Patagonia; purity of the
salt; its origin.
The space between the Cordillera and the coast of Chile is on a rude
average from eighty to above one hundred miles in width; it is formed,
either of an almost continuous mass of mountains, or more commonly of
several nearly parallel ranges, separated by plains; in the more
southern parts of this province the mountains are quite subordinate to
the plains; in the northern part the mountains predominate.
The basin-like plains at the foot of the Cordillera are in several
respects remarkable; that on which the capital of Chile stands is
fifteen miles in width, in an east and west line, and of much greater
length in a north and south line; it stands 1,750 feet above the sea;
its surface appears smooth, but really falls and rises in wide gentle
undulations, the hollows corresponding with the main valleys of the
Cordillera: the striking manner in which it abruptly comes up to the
foot of this great range has been remarked by every author since the
time of Molina. (This plain is partially separated into two basins by a
range of hills; the southern half, according to Meyen (“Reise um Erde”
Th. 1 s. 274), falls in height, by an abrupt step, of between fifteen
and twenty feet.) Near the Cordillera it is composed of a stratified
mass of pebbles of all sizes, occasionally including rounded boulders:
near its western boundary, it consists of reddish sandy clay,
containing some pebbles and numerous fragments of pumice, and sometimes
passes into pure sand or into volcanic ashes. At Podaguel, on this
western side of the plain, beds of sand are capped by a calcareous
tuff, the uppermost layers being generally hard and substalagmitic, and
the lower ones white and friable, both together precisely resembling
the beds at Coquimbo, which contain recent marine shells. Abrupt, but
rounded, hummocks of rock rise out of this plain: those of Sta. Lucia
and S. Cristoval are formed of greenstone-porphyry almost entirely
denuded of its original covering of porphyritic claystone breccia; on
their summits, many fragments of rock (some of them kinds not found in
situ) are coated and united together by a white, friable, calcareous
tuff, like that found at Podaguel. When this matter was deposited on
the summit of S. Cristoval, the water must have stood 946 feet above
the surface of the surrounding plain. (Or 2,690 feet above the sea, as
measured barometrically by Mr. Eck. This tuff appears to the eye nearly
pure; but when placed in acid it leaves a considerable residue of sand
and broken crystals, apparently of feldspar. Dr. Meyen (“Reise” Th. 1
s. 269) says he found a similar substance on the neighbouring hill of
Dominico (and I found it also on the Cerro Blanco), and he attributes
it to the weathering of the stone. In some places which I examined, its
bulk put this view of its origin quite out of the question; and I
should much doubt whether the decomposition of a porphyry would, in any
case, leave a crust chiefly composed of carbonate of lime. The white
crust, which is commonly seen on weathered feldspathic rocks, does not
appear to contain any free carbonate of lime.)
To the south this basin-like plain contracts, and rising scarcely
perceptibly with a smooth surface, passes through a remarkable level
gap in the mountains, forming a true land-strait, and called the
Angostura. It then immediately expands into a second basin-formed
plain: this again to the south contracts into another land-strait, and
expands into a third basin, which, however, falls suddenly in level
about forty feet. This third basin, to the south, likewise contracts
into a strait, and then again opens into the great plain of San
Fernando, stretching so far south that the snowy peaks of the distant
Cordillera are seen rising above its horizon as above the sea. These
plains, near the Cordillera, are generally formed of a thick stratified
mass of shingle (The plain of San Fernando has, according to MM. Meyen
and Gay “Reise” etc. Th. 1 ss. 295 and 298, near the Cordillera, an
upper step-formed plain of clay, on the surface of which they found
numerous blocks of rocks, from two to three feet long, either lying
single or piled in heaps, but all arranged in nearly straight lines.);
in other parts, of a red sandy clay, often with an admixture of
pumiceous matter. Although these basins are connected together like a
necklace, in a north and south line, by smooth land-straits, the
streams which drain them do not all flow north and south, but mostly
westward, through breaches worn in the bounding mountains; and in the
case of the second basin, or that of Rancagua, there are two distinct
breaches. Each basin, moreover, is not drained singly; thus, to give
the most striking instance, but not the only one, in proceeding
southward over the plain of Rancagua, we first find the water flowing
northward to and through the northern land-strait; then, without
crossing any marked ridge or watershed, we see it flowing
south-westward towards the northern one of the two breaches in the
western mountainous boundary; and lastly, again without any ridge, it
flows towards the southern breach in these same mountains. Hence the
surface of this one basin-like plain, appearing to the eye so level,
has been modelled with great nicety, so that the drainage, without any
conspicuous watersheds, is directed towards three openings in the
encircling mountains. ((It appears from Captain Herbert’s account of
the Diluvium of the Himalaya, “Gleanings of Science” Calcutta volume 2
page 164, that precisely similar remarks apply to the drainage of the
plains or valleys between those great mountains.) The streams flowing
from the southern basin-like plains, after passing through the breaches
to the west, unite and form the river Rapel, which enters the Pacific
near Navidad. I followed the southernmost branch of this river, and
found that the basin or plain of San Fernando is continuously and
smoothly united with those plains, which were described in the Second
Chapter, as being worn near the coast into successive cave-eaten
escarpments, and still nearer to the coast, as being strewed with
upraised recent marine remains.
I might have given descriptions of numerous other plains of the same
general form, some at the foot of the Cordillera, some near the coast,
and some halfway between these points. I will allude only to one other,
namely, the plain of Uspallata, lying on the eastern or opposite side
of the Cordillera, between that great range and the parallel lower
range of Uspallata. According to Miers, its surface is 6,000 feet above
the level of the sea: it is from ten to fifteen miles in width, and is
said to extend with an unbroken surface for 180 miles northwards: it is
drained by two rivers passing through breaches in the mountains to the
east. On the banks of the River Mendoza it is seen to be composed of a
great accumulation of stratified shingle, estimated at 400 feet in
thickness. In general appearance, and in numerous points of structure,
this plain closely resembles those of Chile.
The origin and manner of formation of the thick beds of gravel, sandy
clay, volcanic detritus, and calcareous tuff, composing these
basin-like plains, is very important; because, as we shall presently
show, they send arms or fringes far up the main valleys of the
Cordillera. Many of the inhabitants believe that these plains were once
occupied by lakes, suddenly drained; but I conceive that the number of
the separate breaches at nearly the same level in the mountains
surrounding them quite precludes this idea. Had not such distinguished
naturalists as MM. Meyen and Gay stated their belief that these
deposits were left by great debacles rushing down from the Cordillera,
I should not have noticed a view, which appears to me from many reasons
improbable in the highest degree—namely, from the vast accumulation of
WELL-ROUNDED PEBBLES—their frequent stratification with layers of
sand—the overlying beds of calcareous tuff—this same substance coating
and uniting the fragments of rock on the hummocks in the plain of
Santiago—and lastly even from the worn, rounded, and much denuded state
of these hummocks, and of the headlands which project from the
surrounding mountains. On the other hand, these several circumstances,
as well as the continuous union of the basins at the foot of the
Cordillera, with the great plain of the Rio Rapel which still retains
the marks of sea-action at various levels, and their general similarity
in form and composition with the many plains near the coast, which are
either similarly marked or are strewed with upraised marine remains,
fully convince me that the mountains bounding these basin-plains were
breached, their islet-like projecting rocks worn, and the loose
stratified detritus forming their now level surfaces deposited, by the
sea, as the land slowly emerged. It is hardly possible to state too
strongly the perfect resemblance in outline between these basin-like,
long, and narrow plains of Chile (especially when in the early morning
the mists hanging low represented water), and the creeks and fiords now
intersecting the southern and western shores of the continent. We can
on this view of the sea, when the land stood lower, having long and
tranquilly occupied the spaces between the mountain-ranges, understand
how the boundaries of the separate basins were breached in more than
one place; for we see that this is the general character of the inland
bays and channels of Tierra del Fuego; we there, also, see in the
sawing action of the tides, which flow with great force in the cross
channels, a power sufficient to keep the breaches open as the land
emerged. We can further see that the waves would naturally leave the
smooth bottom of each great bay or channel, as it became slowly
converted into land, gently inclined to as many points as there were
mouths, through which the sea finally retreated, thus forming so many
watersheds, without any marked ridges, on a nearly level surface. The
absence of marine remains in these high inland plains cannot be
properly adduced as an objection to their marine origin: for we may
conclude, from shells not being found in the great shingle beds of
Patagonia, though copiously strewed on their surfaces, and from many
other analogous facts, that such deposits are eminently unfavourable
for the embedment of such remains; and with respect to shells not being
found strewed on the surface of these basin-like plains, it was shown
in the last chapter that remains thus exposed in time decay and
disappear.
(FIGURE 13. SECTION OF THE PLAIN AT THE EASTERN FOOT OF THE CHILEAN
CORDILLERA.
From Cordillera (left) through Talus-plain and Level surface, 2,700
feet above sea, to Gravel terraces (right).)
I observed some appearances on the plains at the eastern and opposite
foot of the Cordillera which are worth notice, as showing that the sea
there long acted at nearly the same level as on the basin-plains of
Chile. The mountains on this eastern side are exceedingly abrupt; they
rise out of a smooth, talus-like, very gentle, slope, from five to ten
miles in width (as represented in Figure 13), entirely composed of
perfectly rounded pebbles, often white-washed with an aluminous
substance like decomposed feldspar. This sloping plain or talus blends
into a perfectly flat space a few miles in width, composed of reddish
impure clay, with small calcareous concretions as in the Pampean
deposit,—of fine white sand with small pebbles in layers,—and of the
above-mentioned white aluminous earth, all interstratified together.
This flat space runs as far as Mendoza, thirty miles northward, and
stands probably at about the same height, namely, 2,700 feet (Pentland
and Miers) above the sea. To the east it is bounded by an escarpment,
eighty feet in height, running for many miles north and south, and
composed of perfectly round pebbles, and loose, white-washed, or
embedded in the aluminous earth: behind this escarpment there is a
second and similar one of gravel. Northward of Mendoza, these
escarpments become broken and quite obliterated; and it does not appear
that they ever enclosed a lake-like area: I conclude, therefore, that
they were formed by the sea, when it reached the foot of the
Cordillera, like the similar escarpments occurring at so many points on
the coasts of Chile and Patagonia.
The talus-like plain slopes up with a smooth surface into the great dry
valleys of the Cordillera. On each hand of the Portillo valley, the
mountains are formed of red granite, mica-slate, and basalt, which all
have suffered a truly astonishing amount of denudation; the gravel in
the valley, as well as on the talus-like plain in front of it, is
composed of these rocks; but at the mouth of the valley, in the middle
(height probably about three thousand five hundred feet above the sea),
a few small isolated hillocks of several varieties of porphyry project,
round which, on all sides, smooth and often white-washed pebbles of
these same porphyries, to the exclusion of all others, extend to a
circumscribed distance. Now, it is difficult to conceive any other
agency, except the quiet and long-continued action of the sea on these
hillocks, which could have rounded and whitewashed the fragments of
porphyry, and caused them to radiate from such small and quite
insignificant centres, in the midst of that vast stream of stones which
has descended from the main Cordillera.
SLOPING TERRACES OF GRAVEL IN THE VALLEYS OF THE CORDILLERA.
(FIGURE 14. GROUND-PLAN OF A BIFURCATING VALLEY IN THE CORDILLERA,
bordered by smooth, sloping gravel-fringes (AA), worn along the course
of the river into cliffs.)
All the main valleys on both flanks of the Chilean Cordillera have
formerly had, or still have, their bottoms filled up to a considerable
thickness by a mass of rudely stratified shingle. In Central Chile the
greater part of this mass has been removed by the torrents;
cliff-bounded fringes, more or less continuous, being left at
corresponding heights on both sides of the valleys. These fringes, or
as they may be called terraces, have a smooth surface, and as the
valleys rise, they gently rise with them: hence they are easily
irrigated, and afford great facilities for the construction of the
roads. From their uniformity, they give a remarkable character to the
scenery of these grand, wild, broken valleys. In width, the fringes
vary much, sometimes being only broad enough for the roads, and
sometimes expanding into narrow plains. Their surfaces, besides gently
rising up the valley, are slightly inclined towards its centre in such
a manner as to show that the whole bottom must once have been filled up
with a smooth and slightly concave mass, as still are the dry
unfurrowed valleys of Northern Chile. Where two valleys unite into one,
these terraces are particularly well exhibited, as is represented in
Figure 14. The thickness of the gravel forming these fringes, on a rude
average, may be said to vary from thirty to sixty or eighty feet; but
near the mouths of the valleys it was in several places from two to
three hundred feet. The amount of matter removed by the torrents has
been immense; yet in the lower parts of the valleys the terraces have
seldom been entirely worn away on either side, nor has the solid
underlying rock been reached: higher up the valleys, the terraces have
frequently been removed on one or the other side, and sometimes on both
sides; but in this latter case they reappear after a short interval on
the line, which they would have held had they been unbroken. Where the
solid rock has been reached, it has been cut into deep and narrow
gorges. Still higher up the valleys, the terraces gradually become more
and more broken, narrower, and less thick, until, at a height of from
seven to nine thousand feet, they become lost, and blended with the
piles of fallen detritus.
I carefully examined in many places the state of the gravel, and almost
everywhere found the pebbles equally and perfectly rounded,
occasionally with great blocks of rock, and generally distinctly
stratified, often with parting seams of sand. The pebbles were
sometimes coated with a white aluminous, and less frequently with a
calcareous, crust. At great heights up the valleys the pebbles become
less rounded; and as the terraces become obliterated, the whole mass
passes into the nature of ordinary detritus. I was repeatedly struck
with the great difference between this detritus high up the valleys,
and the gravel of the terraces low down, namely, in the greater number
of the quite angular fragments in the detritus,—in the unequal degree
to which the other fragments have been rounded,—in the quantity of
associated earth,—in the absence of stratification,—and in the
irregularity of the upper surfaces. This difference was likewise well
shown at points low down the valleys, where precipitous ravines,
cutting through mountains of highly coloured rock, have thrown down
wide, fan- shaped accumulations of detritus on the terraces: in such
cases, the line of separation between the detritus and the terrace
could be pointed out to within an inch or two; the detritus consisting
entirely of angular and only partially rounded fragments of the
adjoining coloured rocks; the stratified shingle (as I ascertained by
close inspection, especially in one case, in the valley of the River
Mendoza) containing only a small proportion of these fragments, and
those few well rounded.
I particularly attended to the appearance of the terraces where the
valleys made abrupt and considerable bends, but I could perceive no
difference in their structure: they followed the bends with their usual
nearly equable inclination. I observed, also, in several valleys, that
wherever large blocks of any rock became numerous, either on the
surface of the terrace or embedded in it, this rock soon appeared
higher up in situ: thus I have noticed blocks of porphyry, of andesitic
syenite, of porphyry and of syenite, alternately becoming numerous, and
in each case succeeded by mountains thus constituted. There is,
however, one remarkable exception to this rule; for along the valley of
the Cachapual, M. Gay found numerous large blocks of white granite,
which does not occur in the neighbourhood. I observed these blocks, as
well as others of andesitic syenite (not occurring here in situ), near
the baths of Cauquenes at a height of between two and three hundred
feet above the river, and therefore quite above the terrace or fringe
which borders that river; some miles up the valleys there were other
blocks at about the same height. I also noticed, at a less height, just
above the terrace, blocks of porphyries (apparently not found in the
immediately impending mountains), arranged in rude lines, as on a
sea-beach. All these blocks were rounded, and though large, not
gigantic, like the true erratic boulders of Patagonia and Fuegia. M.
Gay states that the granite does not occur in situ within a distance of
twenty leagues (“Annales des Science Nat. “ 1 series tome 28. M. Gay,
as I was informed, penetrated the Cordillera by the great oblique
valley of Los Cupressos, and not by the most direct line.); I suspect,
for several reasons, that it will ultimately be found at a much less
distance, though certainly not in the immediate neighbourhood. The
boulders found by MM. Meyen and Gay on the upper plain of San Fernando
(mentioned in a previous note) probably belong to this same class of
phenomena.
These fringes of stratified gravel occur along all the great valleys of
the Cordillera, as well as along their main branches; they are
strikingly developed in the valleys of the Maypu, Mendoza, Aconcagua,
Cachapual, and according to Meyen, in the Tinguirica. (“Reise” etc. Th.
1 s. 302.) In the valleys, however, of Northern Chile, and in some on
the eastern flank of the Cordillera, as in the Portillo Valley, where
streams have never flowed, or are quite insignificant in volume, the
presence of a mass of stratified gravel can be inferred only from the
smooth slightly concave form of the bottom. One naturally seeks for
some explanation of so general and striking a phenomenon; that the
matter forming the fringes along the valleys, or still filling up their
entire beds, has not fallen from the adjoining mountains like common
detritus, is evident from the complete contrast in every respect
between the gravel and the piles of detritus, whether seen high up the
valleys on their sides, or low down in front of the more precipitous
ravines; that the matter has not been deposited by debacles, even if we
could believe in debacles having rushed down EVERY valley, and all
their branches, eastward and westward from the central pinnacles of the
Cordillera, we must admit from the following reasons,—from the distinct
stratification of the mass,—its smooth upper surface,—the well-rounded
and sometimes encrusted state of the pebbles, so different from the
loose debris on the mountains,—and especially from the terraces
preserving their uniform inclination round the most abrupt bends. To
suppose that as the land now stands, the rivers deposited the shingle
along the course of every valley, and all their main branches, appears
to me preposterous, seeing that these same rivers not only are now
removing and have removed much of this deposit, but are everywhere
tending to cut deep and narrow gorges in the hard underlying rocks.
I have stated that these fringes of gravel, the origin of which are
inexplicable on the notion of debacles or of ordinary alluvial action,
are directly continuous with the similarly-composed basin-like plains
at the foot of the Cordillera, which, from the several reasons before
assigned, I cannot doubt were modelled by the agency of the sea. Now if
we suppose that the sea formerly occupied the valleys of the Chilean
Cordillera, in precisely the same manner as it now does in the more
southern parts of the continent, where deep winding creeks penetrate
into the very heart of, and in the case of Obstruction Sound quite
through, this great range; and if we suppose that the mountains were
upraised in the same slow manner as the eastern and western coasts have
been upraised within the recent period, then the origin and formation
of these sloping, terrace-like fringes of gravel can be simply
explained. For every part of the bottom of each valley will, on this
view, have long stood at the head of a sea creek, into which the then
existing torrents will have delivered fragments of rocks, where, by the
action of the tides, they will have been rolled, sometimes encrusted,
rudely stratified, and the whole surface levelled by the blending
together of the successive beach lines. (Sloping terraces of precisely
similar structure have been described by me “Philosophical
Transactions” 1839 page 58, in the valleys of Lochaber in Scotland,
where, at higher levels, the parallel roads of Glen Roy show the marks
of the long and quiet residence of the sea. I have no doubt that these
sloping terraces would have been present in the valleys of most of the
European ranges, had not every trace of them, and all wrecks of
sea-action, been swept away by the glaciers which have since occupied
them. I have shown that this is the case with the mountains (“London
and Edinburgh Philosophical Journal” volume 21 page 187) of North
Wales.) As the land rose, the torrents in every valley will have tended
to have removed the matter which just before had been arrested on, or
near, the beach-lines; the torrents, also, having continued to gain in
force by the continued elevation increasing their total descent from
their sources to the sea. This slow rising of the Cordillera, which
explains so well the otherwise inexplicable origin and structure of the
terraces, judging from all known analogies, will probably have been
interrupted by many periods of rest; but we ought not to expect to find
any evidence of these periods in the structure of the gravel- terraces:
for, as the waves at the heads of deep creeks have little erosive
power, so the only effect of the sea having long remained at the same
level will be that the upper parts of the creeks will have become
filled up at such periods to the level of the water with gravel and
sand; and that afterwards the rivers will have thrown down on the
filled-up parts a talus of similar matter, of which the inclination (as
at the head of a partially filled-up lake) will have been determined by
the supply of detritus, and the force of the stream. (I have attempted
to explain this process in a more detailed manner, in a letter to Mr.
Maclaren, published in the “Edinburgh New Philosophical Journal” volume
35 page 288.) Hence, after the final conversion of the creeks into
valleys, almost the only difference in the terraces at those points at
which the sea stood long, will be a somewhat more gentle inclination,
with river-worn instead of sea-worn detritus on the surface.
I know of only one difficulty on the foregoing view, namely, the far-
transported blocks of rock high on the sides of the valley of the
Cachapual: I will not attempt any explanation of this phenomenon, but I
may state my belief that a mountain-ridge near the Baths of Cauquenes
has been upraised long subsequently to all the other ranges in the
neighbourhood, and that when this was effected the whole face of the
country must have been greatly altered. In the course of ages,
moreover, in this and other valleys, events may have occurred like, but
even on a grander scale than, that described by Molina, when a slip
during the earthquake of 1762 banked up for ten days the great River
Lontue, which then bursting its barrier “inundated the whole country,”
and doubtless transported many great fragments of rock. (“Compendio de
la Hist.” etc. etc. tome 1 page 30. M. Brongniart, in his report on M.
Gay’s labours “Annales des Sciences” 1833, considers that the boulders
in the Cachapual belong to the same class with the erratic boulders of
Europe. As the blocks which I saw are not gigantic, and especially as
they are not angular, and as they have not been transported fairly
across low spaces or wide valleys, I am unwilling to class them with
those which, both in the northern and southern hemisphere “Geological
Transactions” volume 6 page 415, have been transported by ice. It is to
be hoped that when M. Gay’s long-continued and admirable labours in
Chile are published, more light will be thrown on this subject.
However, the boulders may have been primarily transported; the final
position of those of porphyry, which have been described as arranged at
the foot of the mountain in rude lines, I cannot doubt, has been due to
the action of waves on a beach. The valley of the Cachapual, in the
part where the boulders occur, bursts through the high ridge of
Cauquenes, which runs parallel to, but at some distance from, the
Cordillera. This ridge has been subjected to excessive violence;
trachytic lava has burst from it, and hot springs yet flow at its base.
Seeing the enormous amount of denudation of solid rock in the upper and
much broader parts of this valley where it enters the Cordillera, and
seeing to what extent the ridge of Cauquenes now protects the great
range, I could not help believing (as alluded to in the text) that this
ridge with its trachytic eruptions had been thrown up at a much later
period than the Cordillera. If this has been the case, the boulders,
after having been transported to a low level by the torrents (which
exhibit in every valley proofs of their power of moving great
fragments), may have been raised up to their present height, with the
land on which they rested.) Finally, notwithstanding this one case of
difficulty, I cannot entertain any doubt, that these terrace-like
fringes, which are continuously united with the basin-shaped plains at
the foot of the Cordillera, have been formed by the arrestment of
river-borne detritus at successive levels, in the same manner as we see
now taking place at the heads of all those many, deep, winding fiords
intersecting the southern coasts. To my mind, this has been one of the
most important conclusions to which my observations on the geology of
South America have led me; for we thus learn that one of the grandest
and most symmetrical mountain-chains in the world, with its several
parallel lines, has been together uplifted in mass between seven and
nine thousand feet, in the same gradual manner as have the eastern and
western coasts within the recent period. (I do not wish to affirm that
all the lines have been uplifted quite equally; slight differences in
the elevation would leave no perceptible effect on the terraces. It
may, however, be inferred, perhaps with one exception, that since the
period when the sea occupied these valleys, the several ranges have not
been dislocated by GREAT and ABRUPT faults or upheavals; for if such
had occurred, the terraces of gravel at these points would not have
been continuous. The one exception is at the lower end of a plain in
the Valle del Yeso (a branch of the Maypu), where, at a great height,
the terraces and valley appear to have been broken through by a line of
upheaval, of which the evidence is plain in the adjoining mountains;
this dislocation, perhaps, occurred AFTER THE ELEVATION of this part of
the valley above the level of the sea. The valley here is almost
blocked up by a pile about one thousand feet in thickness, formed, as
far as I could judge, from three sides, entirely, or at least in chief
part, of gravel and detritus. On the south side, the river has cut
quite through this mass; on the northern side, and on the very summit,
deep ravines, parallel to the line of the valley, are worn, as if the
drainage from the valley above had passed by these two lines before
following its present course.)
FORMATION OF VALLEYS.
The bulk of solid rock which has been removed in the lower parts of the
valleys of the Cordillera has been enormous. It is only by reflecting
on such cases as that of the gravel beds of Patagonia, covering so many
thousand square leagues of surface, and which, if heaped into a ridge,
would form a mountain-range almost equal to the Cordillera, that the
amount of denudation becomes credible. The valleys within this range
often follow anticlinal but rarely synclinal lines; that is, the strata
on the two sides more often dip from the line of valley than towards
it. On the flanks of the range, the valleys most frequently run neither
along anticlinal nor synclinal axes, but along lines of flexure or
faults: that is, the strata on both sides dip in the same direction,
but with different, though often only slightly different, inclinations.
As most of the nearly parallel ridges which together form the
Cordillera run approximately north and south, the east and west valleys
cross them in zig-zag lines, bursting through the points where the
strata have been least inclined. No doubt the greater part of the
denudation was affected at the periods when tidal- creeks occupied the
valleys, and when the outer flanks of the mountains were exposed to the
full force of an open ocean. I have already alluded to the power of the
tidal action in the channels connecting great bays; and I may here
mention that one of the surveying vessels in a channel of this kind,
though under sail, was whirled round and round by the force of the
current. We shall hereafter see, that of the two main ridges forming
the Chilean Cordillera, the eastern and loftiest one owes the greater
part of its ANGULAR upheaval to a period subsequent to the elevation of
the western ridge; and it is likewise probable that many of the other
parallel ridges have been angularly upheaved at different periods;
consequently many parts of the surfaces of these mountains must
formerly have been exposed to the full force of the waves, which, if
the Cordillera were now sunk into the sea, would be protected by
parallel chains of islands. The torrents in the valleys certainly have
great power in wearing the rocks; as could be told by the dull rattling
sound of the many fragments night and day hurrying downwards; and as
was attested by the vast size of certain fragments, which I was assured
had been carried onwards during floods; yet we have seen in the lower
parts of the valleys, that the torrents have seldom removed all the
sea-checked shingle forming the terraces, and have had time since the
last elevation in mass only to cut in the underlying rocks, gorges,
deep and narrow, but quite insignificant in dimensions compared with
the entire width and depth of the valleys.
Along the shores of the Pacific, I never ceased during my many and long
excursions to feel astonished at seeing every valley, ravine, and even
little inequality of surface, both in the hard granitic and soft
tertiary districts, retaining the exact outline, which they had when
the sea left their surfaces coated with organic remains. When these
remains shall have decayed, there will be scarcely any difference in
appearance between this line of coast-land and most other countries,
which we are accustomed to believe have assumed their present features
chiefly through the agency of the weather and fresh-water streams. In
the old granitic districts, no doubt it would be rash to attribute all
the modifications of outline exclusively to the sea-action; for who can
say how often this lately submerged coast may not previously have
existed as land, worn by running streams and washed by rain? This
source of doubt, however, does not apply to the districts superficially
formed of the modern tertiary deposits. The valleys worn by the sea,
through the softer formations, both on the Atlantic and Pacific sides
of the continent, are generally broad, winding, and flat-bottomed: the
only district of this nature now penetrated by arms of the sea, is the
island of Chiloe.
Finally, the conclusion at which I have arrived, with respect to the
relative powers of rain and sea water on the land, is, that the latter
is far the most efficient agent, and that its chief tendency is to
widen the valleys; whilst torrents and rivers tend to deepen them, and
to remove the wreck of the sea’s destroying action. As the waves have
more power, the more open and exposed the space may be, so will they
always tend to widen more and more the mouths of valleys compared with
their upper parts: hence, doubtless, it is, that most valleys expand at
their mouths,—that part, at which the rivers flowing in them, generally
have the least wearing power.
When reflecting on the action of the sea on the land at former levels,
the effect of the great waves, which generally accompany earthquakes,
must not be overlooked: few years pass without a severe earthquake
occurring on some part of the west coast of South America; and the
waves thus caused have great power. At Concepcion, after the shock of
1835, I saw large slabs of sandstone, one of which was six feet long,
three in breadth, and two in thickness, thrown high up on the beach;
and from the nature of the marine animals still adhering to it, it must
have been torn up from a considerable depth. On the other hand, at
Callao, the recoil-wave of the earthquake of 1746 carried great masses
of brickwork, between three and four feet square, some way out seaward.
During the course of ages, the effect thus produced at each successive
level, cannot have been small; and in some of the tertiary deposits on
this line of coast, I observed great boulders of granite and other
neighbouring rocks, embedded in fine sedimentary layers, the
transportal of which, except by the means of earthquake-waves, always
appeared to me inexplicable.
SUPERFICIAL SALINE DEPOSITS.
This subject may be here conveniently treated of: I will begin with the
most interesting case, namely, the superficial saline beds near Iquique
in Peru. The porphyritic mountains on the coast rise abruptly to a
height of between one thousand nine hundred and three thousand feet:
between their summits and an inland plain, on which the celebrated
deposit of nitrate of soda lies, there is a high undulatory district,
covered by a remarkable superficial saliferous crust, chiefly composed
of common salt, either in white, hard, opaque nodules, or mingled with
sand, in this latter case forming a compact sandstone. This saliferous
superficial crust extends from the edge of the coast-escarpment, over
the whole face of the country; but never attains, as I am assured by
Mr. Bollaert (long resident here) any great thickness. Although a very
slight shower falls only at intervals of many years, yet small
funnel-shaped cavities show that the salt has been in some parts
dissolved. (It is singular how slowly, according to the observations of
M. Cordier on the salt-mountain of Cardona in Spain “Ann. des Mines,
Translation of Geolog. Mem.” by De la Beche page 60, salt is dissolved,
where the amount of rain is supposed to be as much as 31.4 of an inch
in the year. It is calculated that only five feet in thickness is
dissolved in the course of a century.) In several places I saw large
patches of sand, quite moist, owing to the quantity of muriate of lime
(as ascertained by Mr. T. Reeks) contained in them. From the compact
salt- cemented sand being either red, purplish, or yellow, according to
the colour of the rocky strata on which it rested, I imagined that this
substance had probably been derived through common alluvial action from
the layers of salt which occur interstratified in the surrounding
mountains (“Journal of Researches” page 444 first edition.): but from
the interesting details given by M. d’Orbigny, and from finding on a
fresh examination of this agglomerated sand, that it is not irregularly
cemented, but consists of thin layers of sand of different tints of
colour, alternating with excessively fine parallel layers of salt, I
conclude that it is not of alluvial origin. M. d’Orbigny observed
analogous saline beds extending from Cobija for five degrees of
latitude northward, and at heights varying from six hundred to nine
hundred feet (“Voyage” etc. page 102. M. d’Orbigny found this deposit
intersected, in many places, by deep ravines, in which there was no
salt. Streams must once, though historically unknown, have flowed in
them; and M. d’Orbigny argues from the presence of undissolved salt
over the whole surrounding country, that the streams must have arisen
from rain or snow having fallen, not in the adjoining country, but on
the now arid Cordillera. I may remark, that from having observed ruins
of Indian buildings in absolutely sterile parts of the Chilian
Cordillera (“Journal” 2nd edition page 357), I am led to believe that
the climate, at a time when Indian man inhabited this part of the
continent, was in some slight degree more humid than it is at
present.): from finding recent sea- shells strewed on these saliferous
beds, and under them, great well-rounded blocks, exactly like those on
the existing beach, he believes that the salt, which is invariably
superficial, has been left by the evaporation of the sea-water. This
same conclusion must, I now believe, be extended to the superficial
saliferous beds of Iquique, though they stand about three thousand feet
above the level of the sea.
Associated with the salt in the superficial beds, there are numerous,
thin, horizontal layers of impure, dirty-white, friable, gypseous and
calcareous tuffs. The gypseous beds are very remarkable, from abounding
with, so as sometimes to be almost composed of, irregular concretions,
from the size of an egg to that of a man’s head, of very hard, compact,
heavy gypsum, in the form of anhydrite. This gypsum contains some
foreign particles of stone; it is stained, judging from its action with
borax, with iron, and it exhales a strong aluminous odour. The surfaces
of the concretions are marked by sharp, radiating, or bifurcating
ridges, as if they had been (but not really) corroded: internally they
are penetrated by branching veins (like those of calcareous spar in the
septaria of the London clay) of pure white anhydrite. These veins might
naturally have been thought to have been formed by subsequent
infiltration, had not each little embedded fragment of rock been
likewise edged in a very remarkable manner by a narrow border of the
same white anhydrite: this shows that the veins must have been formed
by a process of segregation, and not of infiltration. Some of the
little included and CRACKED fragments of foreign rock are penetrated by
the anhydrite, and portions have evidently been thus mechanically
displaced: at St. Helena, I observed that calcareous matter, deposited
by rain water, also had the power to separate small fragments of rock
from the larger masses. (“Volcanic Islands” etc. page 87.) I believe
the superficial gypseous deposit is widely extended: I received
specimens of it from Pisagua, forty miles north of Iquique, and
likewise from Arica, where it coats a layer of pure salt. M. d’Orbigny
found at Cobija a bed of clay, lying above a mass of upraised recent
shells, which was saturated with sulphate of soda, and included thin
layers of fibrous gypsum. (“Voyage Geolog.” etc. page 95.) These widely
extended, superficial, beds of salt and gypsum, appear to me an
interesting geological phenomenon, which could be presented only under
a very dry climate.
The plain or basin, on the borders of which the famous bed of nitrate
of soda lies, is situated at the distance of about thirty miles from
the sea, being separated from it by the saliferous district just
described. It stands at a height of 3,300 feet; its surface is level,
and some leagues in width; it extends forty miles northward, and has a
total length (as I was informed by Mr. Belford Wilson, the
Consul-General at Lima) of 420 miles. In a well near the works,
thirty-six yards in depth, sand, earth, and a little gravel were found:
in another well, near Almonte, fifty yards deep, the whole consisted,
according to Mr. Blake, of clay, including a layer of sand two feet
thick, which rested on fine gravel, and this on coarse gravel, with
large rounded fragments of rock. (See an admirable paper “Geological
and Miscellaneous Notices of Tarapaca” in “Silliman’s American Journal”
volume 44 page 1.) In many parts of this now utterly desert plain,
rushes and large prostrate trees in a hardened state, apparently
Mimosas, are found buried, at a depth from three to six feet; according
to Mr. Blake, they have all fallen to the south-west. The bed of
nitrate of soda is said to extend for forty to fifty leagues along the
western margin of the plain, but is not found in its central parts: it
is from two to three feet in thickness, and is so hard that it is
generally blasted with gunpowder; it slopes gently upwards from the
edge of the plain to between ten and thirty feet above its level. It
rests on sand in which, it is said, vegetable remains and broken shells
have been found; shells have also been found, according to Mr. Blake,
both on and in the nitrate of soda. It is covered by a superficial mass
of sand, containing nodules of common salt, and, as I was assured by a
miner, much soft gypseous matter, precisely like that in the
superficial crust already described: certainly this crust, with its
characteristic concretions of anhydrite, comes close down to the edge
of the plain.
The nitrate of soda varies in purity in different parts, and often
contains nodules of common salt. According to Mr. Blake, the proportion
of nitrate of soda varies from 20 to 75 per cent. An analysis by Mr. A.
Hayes, of an average specimen, gave:—
Nitrate of Soda.... 64.98
Sulphate of Soda.... 3.00
Chloride of Soda... 28.69
Iodic Salts......... 0.63
Shells and Marl..... 2.60
99.90
The “mother-water” at some of the refineries is very rich in iodic
salts, and is supposed to contain much muriate of lime. (“Literary
Gazette” 1841 page 475.) In an unrefined specimen brought home by
myself, Mr. T. Reeks has ascertained that the muriate of lime is very
abundant. With respect to the origin of this saline mass, from the
manner in which the gently inclined, compact bed follows for so many
miles the sinuous margin of the plain, there can be no doubt that it
was deposited from a sheet of water: from the fragments of embedded
shells, from the abundant iodic salts, from the superficial saliferous
crust occurring at a higher level and being probably of marine origin,
and from the plain resembling in form those of Chile and that of
Uspallata, there can be little doubt that this sheet of water was, at
least originally, connected with the sea. (From an official document,
shown me by Mr. Belford Wilson, it appears that the first export of
nitrate of soda to Europe was in July 1830, on French account, in a
British ship:—
In year, the entire export was in Quintals.
1830............................ 17,300
1831............................ 40,885
1832............................ 51,400
1833............................ 91,335
1834........................... 149,538
The Spanish quintal nearly equals 100 English pounds.)
THIN, SUPERFICIAL, SALINE INCRUSTATIONS.
These saline incrustations are common in many parts of America:
Humboldt met with them on the tableland of Mexico, and the Jesuit
Falkner and other authors state that they occur at intervals over the
vast plains extending from the mouth of the Plata to Rioja and
Catamarca. (Azara “Travels” volume 1 page 55, considers that the Parana
is the eastern boundary of the saliferous region; but I heard of
“salitrales” in the Province of Entre Rios.) Hence it is that during
droughts, most of the streams in the Pampas are saline. I nowhere met
with these incrustations so abundantly as near Bahia Blanca: square
miles of the mud-flats, which near that place are raised only a few
feet above the sea, just enough to protect them from being overflowed,
appear, after dry weather, whiter than the ground after the thickest
hoar-frost. After rain the salts disappear, and every puddle of water
becomes highly saline; as the surface dries, the capillary action draws
the moisture up pieces of broken earth, dead sticks, and tufts of
grass, where the salt effloresces. The incrustation, where thickest,
does not exceed a quarter of an inch. M. Parchappe has analysed it (M.
d’Orbigny “Voyage” etc. Part. Hist. tome 1 page 664.); and finds that
the specimens collected at the extreme head of the low plain, near the
River Manuello, consist of 93 per cent of sulphate of soda, and 7 of
common salt; whilst the specimens taken close to the coast contain only
63 per cent of the sulphate, and 37 of the muriate of soda. This
remarkable fact, together with our knowledge that the whole of this low
muddy plain has been covered by the sea within the recent period, must
lead to the suspicion that the common salt, by some unknown process,
becomes in time changed into the sulphate. Friable, calcareous matter
is here abundant, and the case of the apparent double decomposition of
the shells and salt on San Lorenzo, should not be forgotten.
The saline incrustations, near Bahia Blanca, are not confined to,
though most abundant on, the low muddy flats; for I noticed some on a
calcareous plain between thirty and forty feet above the sea, and even
a little occurs in still higher valleys. Low alluvial tracts in the
valleys of the Rivers Negro and Colorado are also encrusted, and in the
latter valley such spaces appeared to be occasionally overflowed by the
river. I observed saline incrustations in some of the valleys of
Southern Patagonia. At Port Desire a low, flat, muddy valley was
thickly incrusted by salts, which on analysis by Mr. T. Reeks, are
found to consist of a mixture of sulphate and muriate of soda, with
carbonate of lime and earthy matter. On the western side of the
continent, the southern coasts are much too humid for this phenomenon;
but in Northern Chile I again met with similar incrustations. On the
hardened mud, in parts of the broad, flat-bottomed valley of Copiapo,
the saline matter encrusts the ground to the thickness of some inches:
specimens, sent by Mr. Bingley to Apothecaries’ Hall for analysis, were
said to consist of carbonate and sulphate of soda. Much sulphate of
soda is found in the desert of Atacama. In all parts of South America,
the saline incrustations occur most frequently on low damp surfaces of
mud, where the climate is rather dry; and these low surfaces have, in
almost every case, been upraised above the level of the sea, within the
recent period.
SALT-LAKES OF PATAGONIA AND LA PLATA.
Salinas, or natural salt-lakes, occur in various formations on the
eastern side of the continent,—in the argillaceo-calcareous deposit of
the Pampas, in the sandstone of the Rio Negro, where they are very
numerous, in the pumiceous and other beds of the Patagonian tertiary
formation, and in small primary districts in the midst of this latter
formation. Port S. Julian is the most southerly point (latitude 49
degrees to 50 degrees) at which salinas are known to occur. (According
to Azara “Travels” volume 1 page 56, there are salt-lakes as far north
as Chaco (latitude 25 degrees), on the banks of the Vermejo. The
salt-lakes of Siberia appear (Pallas “Travels” English Translation
volume 1 page 284) to occur in very similar depressions to those of
Patagonia.) The depressions, in which these salt-lakes lie, are from a
few feet to sixty metres, as asserted by M. d’Orbigny, below the
surface of the surrounding plains (“Voyage Geolog.” page 63.); and,
according to this same author, near the Rio Negro they all trend,
either in the N.E. and S.W. or in E. and W. lines, coincident with the
general slope of the plain. These depressions in the plain generally
have one side lower than the others, but there are no outlets for
drainage. Under a less dry climate, an outlet would soon have been
formed, and the salt washed away. The salinas occur at different
elevations above the sea; they are often several leagues in diameter;
they are generally very shallow, but there is a deep one in a
quartz-rock formation near C. Blanco. In the wet season, the whole, or
a part, of the salt is dissolved, being redeposited during the
succeeding dry season. At this period the appearance of the snow-white
expanse of salt crystallised in great cubes, is very striking. In a
large salina, northward of the Rio Negro, the salt at the bottom,
during the whole year, is between two and three feet in thickness.
The salt rests almost always on a thick bed of black muddy sand, which
is fetid, probably from the decay of the burrowing worms inhabiting it.
(Professor Ehrenberg examined some of this muddy sand, but was unable
to find in it any infusoria.) In a salina, situated about fifteen miles
above the town of El Carmen on the Rio Negro, and three or four miles
from the banks of that river, I observed that this black mud rested on
gravel with a calcareous matrix, similar to that spread over the whole
surrounding plains: at Port S. Julian the mud, also, rested on the
gravel: hence the depressions must have been formed anteriorly to, or
contemporaneously with, the spreading out of the gravel. I was informed
that one small salina occurs in an alluvial plain within the valley of
the Rio Negro, and therefore its origin must be subsequent to the
excavation of that valley. When I visited the salina, fifteen miles
above the town, the salt was beginning to crystallise, and on the muddy
bottom there were lying many crystals, generally placed crossways of
sulphate of soda (as ascertained by Mr. Reeks), and embedded in the
mud, numerous crystals of sulphate of lime, from one to three inches in
length: M. d’Orbigny states that some of these crystals are acicular
and more than even nine inches in length (“Voyage Geolog.” page 64.);
others are macled and of great purity: those I found all contained some
sand in their centres. As the black and fetid sand overlies the gravel,
and that overlies the regular tertiary strata, I think there can be no
doubt that these remarkable crystals of sulphate of lime have been
deposited from the waters of the lake. The inhabitants call the
crystals of selenite, the padre del sal, and those of the sulphate of
soda, the madre del sal; they assured me that both are found under the
same circumstances in several of the neighbouring salinas; and that the
sulphate of soda is annually dissolved, and is always crystallised
before the common salt on the muddy bottom. (This is what might have
been expected; for M. Ballard asserts “Acad. des Sciences” October 7,
1844, that sulphate of soda is precipitated from solution more readily
from water containing muriate of soda in excess, than from pure water.)
The association of gypsum and salt in this case, as well as in the
superficial deposits of Iquique, appears to me interesting, considering
how generally these substances are associated in the older stratified
formations.
Mr. Reeks has analysed for me some of the salt from the salina near the
Rio Negro; he finds it composed entirely of chloride of sodium, with
the exception of 0.26 of sulphate of lime and of 0.22 of earthy matter:
there are no traces of iodic salts. Some salt from the salina
Chiquitos, in the Pampean formation, is equally pure. It is a singular
fact, that the salt from these salinas does not serve so well for
preserving meat, as sea-salt from the Cape de Verde Islands; and a
merchant at Buenos Ayres told me that he considered it as 50 per cent
less valuable. The purity of the Patagonian salt, or absence from it of
those other saline bodies found in all sea- water, is the only
assignable cause for this inferiority; a conclusion which is supported
by the fact lately ascertained, that those salts answer best for
preserving cheese which contain most of the deliquescent chlorides.
(“Horticultural and Agricultural Gazette” 1845 page 93.) (It would
probably well answer for the merchants of Buenos Ayres (considering the
great consumption there of salt for preserving meat) to import the
deliquescent chlorides to mix with the salt from the salinas: I may
call attention to the fact, that at Iquique, a large quantity of
muriate of lime, left in the MOTHER-WATER during the refinement of the
nitrate of soda, is annually thrown away.)
With respect to the origin of the salt in the salinas, the foregoing
analysis seems opposed to the view entertained by M. d’Orbigny and
others, and which seems so probable considering the recent elevation of
this line of coast, namely, that it is due to the evaporation of
sea-water and to the drainage from the surrounding strata impregnated
with sea-salt. I was informed (I know not whether accurately) that on
the northern side of the salina on the Rio Negro, there is a small
brine spring which flows at all times of the year: if this be so, the
salt in this case at least, probably is of subterranean origin. It at
first appears very singular that fresh water can often be procured in
wells, and is sometimes found in small lakes, quite close to these
salinas. (Sir W. Parish states “Buenos Ayres” etc. pages 122 and 170,
that this is the case near the great salinas westward of the S.
Ventana. I have seen similar statements in an ancient MS. Journal
lately published by S. Angelis. At Iquique, where the surface is so
thickly encrusted with saline matter, I tasted water only slightly
brackish, procured in a well thirty-six yards deep; but here one feels
less surprise at its presence, as pure water might percolate under
ground from the not very distant Cordillera.) I am not aware that this
fact bears particularly on the origin of the salt; but perhaps it is
rather opposed to the view of the salt having been washed out of the
surrounding superficial strata, but not to its having been the residue
of sea-water, left in depressions as the land was slowly elevated.
CHAPTER IV.
ON THE FORMATIONS OF THE PAMPAS.
Mineralogical constitution.—Microscopical structure.—Buenos Ayres,
shells embedded in tosca-rock.—Buenos Ayres to the Colorado.—San
Ventana.—Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta,
shells, bones and infusoria of; co-existence of the recent shells and
extinct mammifers.—Buenos Ayres to Santa Fé.—Skeletons of
Mastodon.—Infusoria.—Inferior marine tertiary strata, their
age.—Horse’s tooth.—BANDA ORIENTAL.—Superficial Pampean
formation.—Inferior tertiary strata, variation of, connected with
volcanic action; Macrauchenia Patachonica at San Julian in Patagonia,
age of, subsequent to living mollusca and to the erratic block period.
SUMMARY.—Area of Pampean formation.—Theories of origin.—Source of
sediment.—Estuary origin.—Contemporaneous with existing
mollusca.—Relations to underlying tertiary strata.—Ancient deposit of
estuary origin.—Elevation and successive deposition of the Pampean
formation.—Number and state of the remains of mammifers; their
habitation, food, extinction, and range.—Conclusion.—Localities in
Pampas at which mammiferous remains have been found.
The Pampean formation is highly interesting from its vast extent, its
disputed origin, and from the number of extinct gigantic mammifers
embedded in it. It has upon the whole a very uniform character:
consisting of a more or less dull reddish, slightly indurated,
argillaceous earth or mud, often, but not always, including in
horizontal lines concretions of marl, and frequently passing into a
compact marly rock. The mud, wherever I examined it, even close to the
concretions, did not contain any carbonate of lime. The concretions are
generally nodular, sometimes rough externally, sometimes
stalactiformed; they are of a compact structure, but often penetrated
(as well as the mud) by hair-like serpentine cavities, and occasionally
with irregular fissures in their centres, lined with minute crystals of
carbonate of lime; they are of white, brown, or pale pinkish tints,
often marked by black dendritic manganese or iron; they are either
darker or lighter tinted than the surrounding mass; they contain much
carbonate of lime, but exhale a strong aluminous odour, and leave, when
dissolved in acids, a large but varying residue, of which the greater
part consists of sand. These concretions often unite into irregular
strata; and over very large tracts of country, the entire mass consists
of a hard, but generally cavernous marly rock: some of the varieties
might be called calcareous tuffs.
Dr. Carpenter has kindly examined under the microscope, sliced and
polished specimens of these concretions, and of the solid marl-rock,
collected in various places between the Colorado and Santa Fe Bajada.
In the greater number, Dr. Carpenter finds that the whole substance
presents a tolerably uniform amorphous character, but with traces of
incipient crystalline metamorphosis; in other specimens he finds
microscopically minute rounded concretions of an amorphous substance
(resembling in size those in oolitic rocks, but not having a concentric
structure), united by a cement which is often crystalline. In some, Dr.
Carpenter can perceive distinct traces of shells, corals, Polythalamia,
and rarely of spongoid bodies. For the sake of comparison, I sent Dr.
Carpenter specimens of the calcareous rock, formed chiefly of fragments
of recent shells, from Coquimbo in Chile: in one of these specimens,
Dr. Carpenter finds, besides the larger fragments, microscopical
particles of shells, and a varying quantity of opaque amorphous matter;
in another specimen from the same bed, he finds the whole composed of
the amorphous matter, with layers showing indications of an incipient
crystalline metamorphosis: hence these latter specimens, both in
external appearance and in microscopical structure, closely resemble
those of the Pampas. Dr. Carpenter informs me that it is well known
that chemical precipitation throws down carbonate of lime in the opaque
amorphous state; and he is inclined to believe that the long-continued
attrition of a calcareous body in a state of crystalline or
semi-crystalline aggregation (as, for instance, in the ordinary shells
of Mollusca, which, when sliced, are transparent) may yield the same
result. From the intimate relations between all the Coquimbo specimens,
I can hardly doubt that the amorphous carbonate of lime in them has
resulted from the attrition and decay of the larger fragments of shell:
whether the amorphous matter in the marly rocks of the Pampas has
likewise thus originated, it would be hazardous to conjecture.
For convenience’ sake, I will call the marly rock by the name given to
it by the inhabitants, namely, Tosca-rock; and the reddish argillaceous
earth, Pampean mud. This latter substance, I may mention, has been
examined for me by Professor Ehrenberg, and the result of his
examination will be given under the proper localities.
I will commence my descriptions at a central spot, namely, at Buenos
Ayres, and thence proceed first southward to the extreme limit of the
deposit, and afterwards northward. The plain on which Buenos Ayres
stands is from thirty to forty feet in height. The Pampean mud is here
of a rather pale colour, and includes small nearly white nodules, and
other irregular strata of an unusually arenaceous variety of
tosca-rock. In a well at the depth of seventy feet, according to
Ignatio Nunez, much tosca-rock was met with, and at several points, at
one hundred feet deep, beds of sand have been found. I have already
given a list of the recent marine and estuary shells found in many
parts on the surface near Buenos Ayres, as far as three or four leagues
from the Plata. Specimens from near Ensenada, given me by Sir W.
Parish, where the rock is quarried just beneath the surface of the
plain, consist of broken bivalves, cemented by and converted into white
crystalline carbonate of lime. I have already alluded, in the first
chapter, to a specimen (also given me by Sir W. Parish) from the A. del
Tristan, in which shells, resembling in every respect the Azara
labiata, d’Orbigny, as far as their worn condition permits of
comparison, are embedded in a reddish, softish, somewhat arenaceous
marly rock: after careful comparison, with the aid of a microscope and
acids, I can perceive no difference between the basis of this rock and
the specimens collected by me in many parts of the Pampas. I have also
stated, on the authority of Sir W. Parish, that northward of Buenos
Ayres, on the highest parts of the plain, about forty feet above the
Plata, and two or three miles from it, numerous shells of the Azara
labiata (and I believe of Venus sinuosa) occur embedded in a stratified
earthy mass, including small marly concretions, and said to be
precisely like the great Pampean deposit. Hence we may conclude that
the mud of the Pampas continued to be deposited to within the period of
this existing estuary shell. Although this formation is of such immense
extent, I know of no other instance of the presence of shells in it.
BUENOS AYRES TO THE RIO COLORADO.
With the exception of a few metamorphic ridges, the country between
these two points, a distance of 400 geographical miles, belongs to the
Pampean formation, and in the southern part is generally formed of the
harder and more calcareous varieties. I will briefly describe my route:
about twenty- five miles S.S.W. of the capital, in a well forty yards
in depth, the upper part, and, as I was assured, the entire thickness,
was formed of dark red Pampean mud without concretions. North of the
River Salado, there are many lakes; and on the banks of one (near the
Guardia) there was a little cliff similarly composed, but including
many nodular and stalactiform concretions: I found here a large piece
of tessellated armour, like that of the Glyptodon, and many fragments
of bones. The cliffs on the Salado consist of pale-coloured Pampean
mud, including and passing into great masses of tosca-rock: here a
skeleton of the Megatherium and the bones of other extinct quadrupeds
(see the list at the end of this chapter) were found. Large quantities
of crystallised gypsum (of which specimens were given me) occur in the
cliffs of this river; and likewise (as I was assured by Mr. Lumb) in
the Pampean mud on the River Chuelo, seven leagues from Buenos Ayres: I
mention this because M. d’Orbigny lays some stress on the supposed
absence of this mineral in the Pampean formation.
Southward of the Salado the country is low and swampy, with tosca-rock
appearing at long intervals at the surface. On the banks, however, of
the Tapalguen (sixty miles south of the Salado) there is a large extent
of tosca-rock, some highly compact and even semi-crystalline, overlying
pale Pampean mud with the usual concretions. Thirty miles further
south, the small quartz-ridge of Tapalguen is fringed on its northern
and southern flank, by little, narrow, flat-topped hills of tosca-rock,
which stand higher than the surrounding plain. Between this ridge and
the Sierra of Guitru-gueyu, a distance of sixty miles, the country is
swampy, with the tosca-rock appearing only in four or five spots: this
sierra, precisely like that of Tapalguen, is bordered by horizontal,
often cliff-bounded, little hills of tosca-rock, higher than the
surrounding plain. Here, also, a new appearance was presented in some
extensive and level banks of alluvium or detritus of the neighbouring
metamorphic rocks; but I neglected to observe whether it was stratified
or not. Between Guitru-gueyu and the Sierra Ventana, I crossed a dry
plain of tosca-rock higher than the country hitherto passed over, and
with small pieces of denuded tableland of the same formation, standing
still higher.
The marly or calcareous beds not only come up nearly horizontally to
the northern and southern foot of the great quartzose mountains of the
Sierra Ventana, but interfold between the parallel ranges. The
superficial beds (for I nowhere obtained sections more than twenty feet
deep) retain, even close to the mountains, their usual character: the
uppermost layer, however, in one place included pebbles of quartz, and
rested on a mass of detritus of the same rock. At the very foot of the
mountains, there were some few piles of quartz and tosca-rock detritus,
including land-shells; but at the distance of only half a mile from
these lofty, jagged, and battered mountains, I could not, to my great
surprise, find on the boundless surface of the calcareous plain even a
single pebble. Quartz- pebbles, however, of considerable size have at
some period been transported to a distance of between forty and fifty
miles to the shores of Bahia Blanca. (Schmidtmeyer “Travels in Chile”
page 150, states that he first noticed on the Pampas, very small bits
of red granite, when fifty miles distant from the southern extremity of
the mountains of Cordova, which project on the plain, like a reef into
the sea.)
The highest peak of the St. Ventana is, by Captain Fitzroy’s
measurement, 3,340 feet, and the calcareous plain at its foot (from
observations taken by some Spanish officers) 840 feet above the
sea-level. (“La Plata” etc. by Sir W. Parish page 146.) On the flanks
of the mountains, at a height of three hundred or four hundred feet
above the plain, there were a few small patches of conglomerate and
breccia, firmly cemented by ferruginous matter to the abrupt and
battered face of the quartz—traces being thus exhibited of ancient
sea-action. The high plain round this range sinks quite insensibly to
the eye on all sides, except to the north, where its surface is broken
into low cliffs. Round the Sierras Tapalguen, Guitru-gueyu, and between
the latter and the Ventana we have seen (and shall hereafter see round
some hills in Banda Oriental), that the tosca-rock forms low, flat-
topped, cliff-bounded hills, higher than the surrounding plains of
similar composition. From the horizontal stratification and from the
appearance of the broken cliffs, the greater height of the Pampean
formation round these primary hills ought not to be altogether or in
chief part attributed to these several points having been uplifted more
energetically than the surrounding country, but to the
argillaceo-calcareous mud having collected round them, when they
existed as islets or submarine rocks, at a greater height, than at the
bottom of the adjoining open sea;—the cliffs having been subsequently
worn during the elevation of the whole country in mass.
Southward of the Ventana, the plain extends farther than the eye can
range; its surface is not very level, having slight depressions with no
drainage exits; it is generally covered by a few feet in thickness of
sandy earth; and in some places, according to M. Parchappe, by beds of
clay two yards thick. (M. d’Orbigny “Voyage” Part Geolog. pages 47,
48.) On the banks of the Sauce, four leagues S.E. of the Ventana, there
is an imperfect section about two hundred feet in height, displaying in
the upper part tosca-rock and in the lower part red Pampean mud. At the
settlement of Bahia Blanca, the uppermost plain is composed of very
compact, stratified tosca-rock, containing rounded grains of quartz
distinguishable by the naked eye: the lower plain, on which the
fortress stands, is described by M. Parchappe as composed of solid
tosca-rock (Ibid.); but the sections which I examined appeared more
like a redeposited mass of this rock, with small pebbles and fragments
of quartz. I shall immediately return to the important sections on the
shores of Bahia Blanca. Twenty miles southward of this place, there is
a remarkable ridge extending W. by N. and E. by S., formed of small,
separate, flat-topped, steep-sided hills, rising between one hundred
and two hundred feet above the Pampean plain at its southern base,
which plain is a little lower than that to the north. The uppermost
stratum in this ridge consists of pale, highly calcareous, compact
tosca-rock, resting (as seen in one place) on reddish Pampean mud, and
this again on a paler kind: at the foot of the ridge, there is a well
in reddish clay or mud. I have seen no other instance of a chain of
hills belonging to the Pampean formation; and as the strata show no
signs of disturbance, and as the direction of the ridge is the same
with that common to all the metamorphic lines in this whole area, I
suspect that the Pampean sediment has in this instance been accumulated
on and over a ridge of hard rocks, instead of, as in the case of the
above-mentioned Sierras, round their submarine flanks. South of this
little chain of tosca-rock, a plain of Pampean mud declines towards the
banks of the Colorado: in the middle a well has been dug in red Pampean
mud, covered by two feet of white, softish, highly calcareous
tosca-rock, over which lies sand with small pebbles three feet in
thickness—the first appearance of that vast shingle formation described
in the First Chapter. In the first section after crossing the Colorado,
an old tertiary formation, namely, the Rio Negro sandstone (to be
described in the next chapter), is met with: but from the accounts
given me by the Gauchos, I believe that at the mouth of the Colorado
the Pampean formation extends a little further southwards.
BAHIA BLANCA.
To return to the shores of this bay. At Monte Hermoso there is a good
section, about one hundred feet in height, of four distinct strata,
appearing to the eye horizontal, but thickening a little towards the
N.W. The uppermost bed, about twenty feet in thickness, consists of
obliquely laminated, soft sandstone, including many pebbles of quartz,
and falling at the surface into loose sand. The second bed, only six
inches thick, is a hard, dark-coloured sandstone. The third bed is
pale-coloured Pampean mud; and the fourth is of the same nature, but
darker coloured, including in its lower part horizontal layers and
lines of concretions of not very compact pinkish tosca-rock. The bottom
of the sea, I may remark, to a distance of several miles from the
shore, and to a depth of between sixty and one hundred feet, was found
by the anchors to be composed of tosca-rock and reddish Pampean mud.
Professor Ehrenberg has examined for me specimens of the two lower
beds, and finds in them three Polygastrica and six Phytolitharia.
(The following list is given in the “Monatsberichten der konig. Akad.
zu Berlin” April 1845:— POLYGASTRICA. Fragilaria rhabdosoma.
Gallionella distans. Pinnularia?
PHYTOLITHARIA. Lithodontium Bursa. Lithodontium furcatum.
Lithostylidium exesum. Lithostylidium rude. Lithostylidium Serra.
Spongolithis Fustis?)
Of these, only one (Spongolithis Fustis?) is a marine form; five of
them are identical with microscopical structures of brackish-water
origin, hereafter to be mentioned, which form a central point in the
Pampean formation. In these two beds, especially in the lower one,
bones of extinct mammifers, some embedded in their proper relative
positions and others single, are very numerous in a small extent of the
cliffs. These remains consist of, first, the head of Ctenomys antiquus,
allied to the living Ctenomys Braziliensis; secondly, a fragment of the
remains of a rodent; thirdly, molar teeth and other bones of a large
rodent, closely allied to, but distinct from, the existing species of
Hydrochoerus, and therefore probably an inhabitant of fresh water;
fourth and fifthly, portions of vertebrae, limbs, ribs, and other bones
of two rodents; sixthly, bones of the extremities of some great
megatheroid quadruped. (See “Fossil Mammalia” page 109 by Professor
Owen, in the “Zoology of the Voyage of the ‘Beagle’;” and Catalogue
page 36 of Fossil Remains in Museum of Royal College of Surgeons.) The
number of the remains of rodents gives to this collection a peculiar
character, compared with those found in any other locality. All these
bones are compact and heavy; many of them are stained red, with their
surfaces polished; some of the smaller ones are as black as jet.
Monte Hermoso is between fifty and sixty miles distant in a S.E. line
from the Ventana, with the intermediate country gently rising towards
it, and all consisting of the Pampean formation. What relation, then,
do these beds, at the level of the sea and under it, bear to those on
the flanks of the Ventana, at the height of 840 feet, and on the flanks
of the other neighbouring sierras, which, from the reasons already
assigned, do not appear to owe their greater height to unequal
elevation? When the tosca- rock was accumulating round the Ventana, and
when, with the exception of a few small rugged primary islands, the
whole wide surrounding plains must have been under water, were the
strata at Monte Hermoso depositing at the bottom of a great open sea,
between eight hundred and one thousand feet in depth? I much doubt
this; for if so, the almost perfect carcasses of the several small
rodents, the remains of which are so very numerous in so limited a
space, must have been drifted to this spot from the distance of many
hundred miles. It appears to me far more probable, that during the
Pampean period this whole area had commenced slowly rising (and in the
cliffs, at several different heights we have proofs of the land having
been exposed to sea-action at several levels), and that tracts of land
had thus been formed of Pampean sediment round the Ventana and the
other primary ranges, on which the several rodents and other quadrupeds
lived, and that a stream (in which perhaps the extinct aquatic
Hydrochoerus lived) drifted their bodies into the adjoining sea, into
which the Pampean mud continued to be poured from the north. As the
land continued to rise, it appears that this source of sediment was cut
off; and in its place sand and pebbles were borne down by stronger
currents, and conformably deposited over the Pampean strata.
(FIGURE 15. SECTION OF BEDS WITH RECENT SHELLS AND EXTINCT MAMMIFERS,
AT PUNTA ALTA IN BAHIA BLANCA. (Showing beds from bottom to top: A, B,
C, D.))
Punta Alta is situated about thirty miles higher up on the northern
side of this same bay: it consists of a small plain, between twenty and
thirty feet in height, cut off on the shore by a line of low cliffs
about a mile in length, represented in Figure 15 with its vertical
scale necessarily exaggerated. The lower bed (A) is more extensive than
the upper ones; it consists of stratified gravel or conglomerate,
cemented by calcareo- arenaceous matter, and is divided by curvilinear
layers of pinkish marl, of which some are precisely like tosca-rock,
and some more sandy. The beds are curvilinear, owing to the action of
currents, and dip in different directions; they include an
extraordinary number of bones of gigantic mammifers and many shells.
The pebbles are of considerable size, and are of hard sandstone, and of
quartz, like that of the Ventana: there are also a few well-rounded
masses of tosca-rock.
The second bed B is about fifteen feet in thickness, but towards both
extremities of the cliff (not included in the diagram) it either thins
out and dies away, or passes insensibly into an overlying bed of
gravel. It consists of red, tough clayey mud, with minute linear
cavities; it is marked with faint horizontal shades of colour; it
includes a few pebbles, and rarely a minute particle of shell: in one
spot, the dermal armour and a few bones of a Dasypoid quadruped were
embedded in it: it fills up furrows in the underlying gravel. With the
exception of the few pebbles and particles of shells, this bed
resembles the true Pampean mud; but it still more closely resembles the
clayey flats (mentioned in the First Chapter) separating the
successively rising parallel ranges of sand-dunes.
The bed C is of stratified gravel, like the lowest one; it fills up
furrows in the underlying red mud, and is sometimes interstratified
with it, and sometimes insensibly passes into it; as the red mud thins
out, this upper gravel thickens. Shells are more numerous in it than in
the lower gravel; but the bones, though some are still present, are
less numerous. In one part, however, where this gravel and the red mud
passed into each other, I found several bones and a tolerably perfect
head of the Megatherium. Some of the large Volutas, though embedded in
the gravel-bed C, were filled with the red mud, including great numbers
of the little recent Paludestrina australis. These three lower beds are
covered by an unconformable mantle D of stratified sandy earth,
including many pebbles of quartz, pumice and phonolite, land and
sea-shells.
M. d’Orbigny has been so obliging as to name for me the twenty species
of Mollusca embedded in the two gravel beds: they consist of:—
1. Volutella angulata, d’Orbigny, “Voyage” Mollusq. and Pal. 2. Voluta
Braziliana, Sol 3. Olicancilleria Braziliensis d’Orbigny. 4.
Olicancilleria auricularia, d’Orbigny. 5. Olivina puelchana, d’Orbigny.
6. Buccinanops cochlidium, d’Orbigny. 7. Buccinanops globulosum,
d’Orbigny. 8. Colombella sertulariarum, d’Orbigny. 9. Trochus
Patagonicus, and var. of ditto, d’Orbigny. 10. Paludestrina Australis,
d’Orbigny. 11. Fissurella Patagonica, d’Orbigny. 12. Crepidula
muricata, Lam. 13. Venus purpurata, Lam. 14. Venus rostrata, Phillippi.
15. Mytilus Darwinianus, d’Orbigny. 16. Nucula semiornata, d’Orbigny.
17. Cardita Patagonica, d’Orbigny. 18. Corbula Patagonica (?),
d’Orbigny. 19. Pecten tethuelchus, d’Orbigny. 20. Ostrea puelchana,
d’Orbigny. 21. A living species of Balanus. 22 and 23. An Astrae and
encrusting Flustra, apparently identical with species now living in the
bay.
All these shells now live on this coast, and most of them in this same
bay. I was also struck with the fact, that the proportional numbers of
the different kinds appeared to be the same with those now cast up on
the beach: in both cases specimens of Voluta, Crepidula, Venus, and
Trochus are the most abundant. Four or five of the species are the same
with the upraised shells on the Pampas near Buenos Ayres. All the
specimens have a very ancient and bleached appearance, and do not emit,
when heated, an animal odour: some of them are changed throughout into
a white, soft, fibrous substance; others have the space between the
external walls, either hollow, or filled up with crystalline carbonate
of lime. (A Bulinus, mentioned in the Introduction to the “Fossil
Mammalia” in the “Zoology of the Voyage of the ‘Beagle’” has so much
fresher an appearance, than the marine species, that I suspect it must
have fallen amongst the others, and been collected by mistake.)
The remains of the extinct mammiferous animals, from the two gravel
beds have been described by Professor Owen in the “Zoology of the
Voyage of the ‘Beagle’:” they consist of, 1st, one nearly perfect head
and three fragments of heads of the Megatherium Cuvierii; 2nd, a lower
jaw of Megalonyx Jeffersonii; 3rd, lower jaw of Mylodon Darwinii; 4th,
fragments of a head of some gigantic Edental quadruped; 5th, an almost
entire skeleton of the great Scelidotherium leptocephalum, with most of
the bones, including the head, vertebrae, ribs, some of the extremities
to the claw- bone, and even, as remarked by Professor Owen, the
knee-cap, all nearly in their proper relative positions; 6th, fragments
of the jaw and a separate tooth of a Toxodon, belonging either to T.
Platensis, or to a second species lately discovered near Buenos Ayres;
7th, a tooth of Equus curvidens; 8th, tooth of a Pachyderm, closely
allied to Palaeotherium, of which parts of the head have been lately
sent from Buenos Ayres to the British Museum; in all probability this
pachyderm is identical with the Macrauchenia Patagonica from Port S.
Julian, hereafter to be referred to. Lastly, and 9thly, in a cliff of
the red clayey bed B, there was a double piece, about three feet long
and two wide, of the bony armour of a large Dasypoid quadruped, with
the two sides pressed nearly close together: as the cliff is now
rapidly washing away, this fossil probably was lately much more
perfect; from between its doubled-up sides, I extracted the middle and
ungual phalanges, united together, of one of the feet, and likewise a
separate phalanx: hence one or more of the limbs must have been
attached to the dermal case, when it was embedded. Besides these
several remains in a distinguishable condition, there were very many
single bones: the greater number were embedded in a space 200 yards
square. The preponderance of the Edental quadrupeds is remarkable; as
is, in contrast with the beds of Monte Hermoso, the absence of Rodents.
Most of the bones are now in a soft and friable condition, and, like
the shells, do not emit when burnt an animal odour. The decayed state
of the bones may be partly owing to their late exposure to the air and
tidal-waves. Barnacles, Serpulae, and corallines are attached to many
of the bones, but I neglected to observe whether these might not have
grown on them since being exposed to the present tidal action (After
having packed up my specimens at Bahia Blanca, this point occurred to
me, and I noted it; but forgot it on my return, until the remains had
been cleaned and oiled: my attention has been lately called to the
subject by some remarks by M. d’Orbigny.); but I believe that some of
the barnacles must have grown on the Scelidotherium, soon after being
deposited, and before being WHOLLY covered up by the gravel. Besides
the remains in the condition here described, I found one single
fragment of bone very much rolled, and as black as jet, so as perfectly
to resemble some of the remains from Monte Hermoso.
Very many of the bones had been broken, abraded, and rolled, before
being embedded. Others, even some of those included in the coarsest
parts of the the now hard conglomerate, still retain all their minutest
prominences perfectly preserved; so that I conclude that they probably
were protected by skin, flesh, or ligaments, whilst being covered up.
In the case of the Scelidotherium, it is quite certain that the whole
skeleton was held together by its ligaments, when deposited in the
gravel in which I found it. Some cervical vertebrae and a humerus of
corresponding size lay so close together, as did some ribs and the
bones of a leg, that I thought that they must originally have belonged
to two skeletons, and not have been washed in single; but as remains
were here very numerous, I will not lay much stress on these two cases.
We have just seen that the armour of the Dasypoid quadruped was
certainly embedded together with some of the bones of the feet.
Professor Ehrenberg has examined for me specimens of the finer matter
from in contact with these mammiferous remains: he finds in them two
Polygastrica, decidedly marine forms; and six Phytolitharia, of which
one is probably marine, and the others either of fresh-water or
terrestrial origin. (“Monatsberichten der Akad. zu Berlin” April 1845.
The list consists of:—
POLYGASTRICA. Gallionella sulcata. Stauroptera aspera? fragm.
PHYTOLITHARIA. Lithasteriscus tuberculatus. Lithostylidium
Clepsammidium. Lithostylidium quadratum. Lithostylidium rude.
Lithostylidium unidentatum. Spongolithis acicularis.)
Only one of these eight microscopical bodies is common to the nine from
Monte Hermoso: but five of them are in common with those from the
Pampean mud on the banks of the Parana. The presence of any fresh-water
infusoria, considering the aridity of the surrounding country, is here
remarkable: the most probable explanation appears to be, that these
microscopical organisms were washed out of the adjoining great Pampean
formation during its denudation, and afterwards redeposited.
We will now see what conclusions may be drawn from the facts above
detailed. It is certain that the gravel-beds and intermediate red mud
were deposited within the period, when existing species of Mollusca
held to each other nearly the same relative proportions as they do on
the present coast. These beds, from the number of littoral species,
must have been accumulated in shallow water; but not, judging from the
stratification of the gravel and the layers of marl, on a beach. From
the manner in which the red clay fills up furrows in the underlying
gravel, and is in some parts itself furrowed by the overlying gravel,
whilst in other parts it either insensibly passes into, or alternates
with, this upper gravel, we may infer several local changes in the
currents, perhaps caused by slight changes, up or down, in the level of
the land. By the elevation of these beds, to which period the alluvial
mantle with pumice-pebbles, land and sea-shells belongs, the plain of
Punta Alta, from twenty to thirty feet in height, was formed. In this
neighbourhood there are other and higher sea-formed plains and lines of
cliffs in the Pampean formation worn by the denuding action of the
waves at different levels. Hence we can easily understand the presence
of rounded masses of tosca-rock in this lowest plain; and likewise, as
the cliffs at Monte Hermoso with their mammiferous remains stand at a
higher level, the presence of the one much-rolled fragment of bone
which was as black as jet: possibly some few of the other much-rolled
bones may have been similarly derived, though I saw only the one
fragment, in the same condition with those from Monte Hermoso. M.
d’Orbigny has suggested that all these mammiferous remains may have
been washed out of the Pampean formation, and afterwards redeposited
together with the recent shells. (“Voyage” Part. Geolog. page 49.)
Undoubtedly it is a marvellous fact that these numerous gigantic
quadrupeds, belonging, with the exception of the Equus curvidens, to
seven extinct genera, and one, namely, the Toxodon, not falling into
any existing family, should have co-existed with Mollusca, all of which
are still living species; but analogous facts have been observed in
North America and in Europe. In the first place, it should not be
overlooked, that most of the co-embedded shells have a more ancient and
altered appearance than the bones. In the second place, is it probable
that numerous bones not hardened by silex or any other mineral, could
have retained their delicate prominences and surfaces perfect if they
had been washed out of one deposit, and re-embedded in another:—this
later deposit being formed of large, hard pebbles, arranged by the
action of currents or breakers in shallow water into variously curved
and inclined layers? The bones which are now in so perfect a state of
preservation, must, I conceive, have been fresh and sound when
embedded, and probably were protected by skin, flesh, or ligaments. The
skeleton of the Scelidotherium indisputably was deposited entire: shall
we say that when held together by its matrix it was washed out of an
old gravel-bed (totally unlike in character to the Pampean formation),
and re-embedded in another gravel-bed, composed (I speak after careful
comparison) of exactly the same kind of pebbles, in the same kind of
cement? I will lay no stress on the two cases of several ribs and bones
of the extremities having APPARENTLY been embedded in their proper
relative position: but will any one be so bold as to affirm that it is
possible, that a piece of the thin tessellated armour of a Dasypoid
quadruped, at least three feet long and two in width, and now so tender
that I was unable with the utmost care to extract a fragment more than
two or three inches square, could have been washed out of one bed, and
re-embedded in another, together with some of the small bones of the
feet, without having been dashed into atoms? We must then wholly reject
M. d’Orbigny’s supposition, and admit as certain, that the
Scelidotherium and the large Dasypoid quadruped, and as highly
probable, that the Toxodon, Megatherium, etc., some of the bones of
which are perfectly preserved, were embedded for the first time, and in
a fresh condition, in the strata in which they were found entombed.
These gigantic quadrupeds, therefore, though belonging to extinct
genera and families, coexisted with the twenty above-enumerated
Mollusca, the barnacle and two corals, still living on this coast. From
the rolled fragment of black bone, and from the plain of Punta Alta
being lower than that of Monte Hermoso, I conclude that the coarse
sub-littoral deposits of Punta Alta, are of subsequent origin to the
Pampean mud of Monte Hermoso; and the beds at this latter place, as we
have seen, are probably of subsequent origin to the high tosca-plain
round the Sierra Ventana: we shall, however, return, at the end of this
chapter, to the consideration of these several stages in the great
Pampean formation.
BUENOS AYRES TO ST. FE BAJADA, IN ENTRE RIOS.
For some distance northward of Buenos Ayres, the escarpment of the
Pampean formation does not approach very near to the Plata, and it is
concealed by vegetation: but in sections on the banks of the Rios
Luxan, Areco, and Arrecifes, I observed both pale and dark reddish
Pampean mud, with small, whitish concretions of tosca; at all these
places mammiferous remains have been found. In the cliffs on the
Parana, at San Nicolas, the Pampean mud contains but little tosca; here
M. d’Orbigny found the remains of two rodents (Ctenomys Bonariensis and
Kerodon antiquus) and the jaw of a Canis: when on the river I could
clearly distinguish in this fine line of cliffs, “horizontal lines of
variation both in tint and compactness.” (I quote these words from my
note-book, as written down on the spot, on account of the general
absence of stratification in the Pampean formation having been insisted
on by M. d’Orbigny as a proof of the diluvial origin of this great
deposit.) The plain northward of this point is very level, but with
some depressions and lakes; I estimated its height at from forty to
sixty feet above the Parana. At the A. Medio the bright red Pampean mud
contains scarcely any tosca-rock; whilst at a short distance the stream
of the Pabon, forms a cascade, about twenty feet in height, over a
cavernous mass of two varieties of tosca-rock; of which one is very
compact and semi- crystalline, with seams of crystallised carbonate of
lime: similar compact varieties are met with on the Salidillo and Seco.
The absolute identity (I speak after a comparison of my specimens)
between some of these varieties, and those from Tapalguen, and from the
ridge south of Bahia Blanca, a distance of 400 miles of latitude, is
very striking.
At Rosario there is but little tosca-rock: near this place I first
noticed at the edge of the river traces of an underlying formation,
which, twenty- five miles higher up in the estancia of Gorodona,
consists of a pale yellowish clay, abounding with concretionary
cylinders of a ferruginous sandstone. This bed, which is probably the
equivalent of the older tertiary marine strata, immediately to be
described in Entre Rios, only just rises above the level of the Parana
when low. The rest of the cliff at Gorodona, is formed of red Pampean
mud, with, in the lower part, many concretions of tosca, some
stalacti-formed, and with only a few in the upper part: at the height
of six feet above the river, two gigantic skeletons of the Mastodon
Andium were here embedded; their bones were scattered a few feet apart,
but many of them still held their proper relative positions: they were
much decayed and as soft as cheese, so that even one of the great molar
teeth fell into pieces in my hand. We here see that the Pampean deposit
contains mammiferous remains close to its base. On the banks of the
Carcarana, a few miles distant, the lowest bed visible was pale Pampean
mud, with masses of tosca-rock, in one of which I found a much decayed
tooth of the Mastodon: above this bed, there was a thin layer almost
composed of small concretions of white tosca, out of which I extracted
a well preserved, but slightly broken tooth of Toxodon Platensis: above
this there was an unusual bed of very soft impure sandstone. In this
neighbourhood I noticed many single embedded bones, and I heard of
others having been found in so perfect a state that they were long used
as gate-posts: the Jesuit Falkner found here the dermal armour of some
gigantic Edental quadruped.
In some of the red mud scraped from a tooth of one of the Mastodons at
Gorodona, Professor Ehrenberg finds seven Polygastrica and thirteen
Phytolitharia, all of them, I believe, with two exceptions, already
known species. (“Monatsberichten der konig. Akad. zu Berlin” April
1845. The list consists of:—
POLYGASTRICA. Campylodiscus clypeus. Coscinodiscus subtilis.
Coscinodiscus al. sp. Eunotia. Gallionella granulata. Himantidium
gracile. Pinnularia borealis.)
Of these twenty, the preponderating number are of fresh-water origin;
only two species of Coscinodiscus and a Spongolithis show the direct
influence of the sea; therefore Professor Ehrenberg arrives at the
important conclusion that the deposit must have been of brackish-water
origin. Of the thirteen Phytolitharia, nine are met with in the two
deposits in Bahia Blanca, where there is evidence from two other
species of Polygastrica that the beds were accumulated in brackish
water. The traces of coral, sponges, and Polythalamia, found by Dr.
Carpenter in the tosca-rock (of which I must observe the greater number
of specimens were from the upper beds in the southern parts of the
formation), apparently show a more purely marine origin.
At ST. FE BAJADA, in Entre Rios, the cliffs, estimated at between sixty
and seventy feet in height, expose an interesting section: the lower
half consists of tertiary strata with marine shells, and the upper half
of the Pampean formation. The lowest bed is an obliquely laminated,
blackish, indurated mud, with distinct traces of vegetable remains. (M.
d’Orbigny “Voyage” Part. Geolog. page 37, has given a detailed
description of this section, but as he does not mention this lowest
bed, it may have been concealed when he was there by the river. There
is a considerable discrepancy between his description and mine, which I
can only account for by the beds themselves varying considerably in
short distances.) Above this there is a thick bed of yellowish sandy
clay, with much crystallised gypsum and many shells of Ostreae,
Pectens, and Arcae: above this there generally comes an arenaceous
crystalline limestone, but there is sometimes interposed a bed, about
twelve feet thick, of dark green, soapy clay, weathering into small
angular fragments. The limestone, where purest, is white, highly
crystalline, and full of cavities: it includes small pebbles of quartz,
broken shells, teeth of sharks, and sometimes, as I was informed, large
bones: it often contains so much sand as to pass into a calcareous
sandstone, and in such parts the great Ostrea Patagonica chiefly
abounds. (Captain Sulivan, R.N., has given me a specimen of this shell,
which he found in the cliffs at Point Cerrito, between twenty and
thirty miles above the Bajada.) In the upper part, the limestone
alternates with layers of fine white sand. The shells included in these
beds have been named for me by M. d’Orbigny: they consist of:—
1. Ostrea Patagonica, d’Orbigny, “Voyage” Part. Pal. 2. Ostrea
Alvarezii, d’Orbigny, “Voyage” Part. Pal. 3. Pecten Paranensis,
d’Orbigny, “Voyage” Part. Pal. 4. Pecten Darwinianus, d’Orbigny,
“Voyage” Part. Pal. 5. Venus Munsterii, d’Orbigny, “Voyage” Pal. 6.
Arca Bonplandiana, d’Orbigny, “Voyage” Pal. 7. Cardium Platense,
d’Orbigny, “Voyage” Pal. 8. Tellina, probably nov. species, but too
imperfect for description.
PHYTOLITHARIA.
Lithasteriscus tuberculatus. Lithodontium bursa. Lithodontium furcatum.
Lithodontium rostratum. Lithostylidium Amphiodon. Lithostylidium
Clepsammidium. Lithostylidium Hamus. Lithostylidium polyedrum.
Lithostylidium quadratum. Lithostylidium rude. Lithostylidium Serra.
Lithostylidium unidentatum. Spongolithis Fustis.
These species are all extinct: the six first were found by M. d’Orbigny
and myself in the formations of the Rio Negro, S. Josef, and other
parts of Patagonia; and therefore, as first observed by M. d’Orbigny,
these beds certainly belong to the great Patagonian formation, which
will be described in the ensuing chapter, and which we shall see must
be considered as a very ancient tertiary one. North of the Bajada, M.
d’Orbigny found, in beds which he considers as lying beneath the strata
here described, remains of a Toxodon, which he has named as a distinct
species from the T. Platensis of the Pampean formation. Much silicified
wood is found on the banks of the Parana (and likewise on the Uruguay),
and I was informed that they come out of these lower beds; four
specimens collected by myself are dicotyledonous.
The upper half of the cliff, to a thickness of about thirty feet,
consists of Pampean mud, of which the lower part is pale-coloured, and
the upper part of a brighter red, with some irregular layers of an
arenaceous variety of tosca, and a few small concretions of the
ordinary kind. Close above the marine limestone, there is a thin
stratum with a concretionary outline of white hard tosca-rock or marl,
which may be considered either as the uppermost bed of the inferior
deposits, or the lowest of the Pampean formation; at one time I
considered this bed as marking a passage between the two formations:
but I have since become convinced that I was deceived on this point. In
the section on the Parana, I did not find any mammiferous remains; but
at two miles distance on the A. Tapas (a tributary of the Conchitas),
they were extremely numerous in a low cliff of red Pampean mud with
small concretions, precisely like the upper bed on the Parana. Most of
the bones were solitary and much decayed; but I saw the dermal armour
of a gigantic Edental quadruped, forming a caldron-like hollow, four or
five feet in diameter, out of which, as I was informed, the almost
entire skeleton had been lately removed. I found single teeth of the
Mastodon Andium, Toxodon Platensis, and Equus curvidens, near to each
other. As this latter tooth approaches closely to that of the common
horse, I paid particular attention to its true embedment, for I did not
at that time know that there was a similar tooth hidden in the matrix
with the other mammiferous remains from Punta Alta. It is an
interesting circumstance, that Professor Owen finds that the teeth of
this horse approach more closely in their peculiar curvature to a
fossil specimen brought by Mr. Lyell from North America, than to those
of any other species of Equus. (Lyell “Travels in North America” volume
1 page 164 and “Proceedings of Geological Society” volume 4 page 39.)
The underlying marine tertiary strata extend over a wide area: I was
assured that they can be traced in ravines in an east and west line
across Entre Rios to the Uruguay, a distance of about 135 miles. In a
S.E. direction I heard of their existence at the head of the R. Nankay;
and at P. Gorda in Banda Oriental, a distance of 170 miles, I found the
same limestone, containing the same fossil shells, lying at about the
same level above the river as at St. Fe. In a southerly direction,
these beds sink in height, for at another P. Gorda in Entre Rios, the
limestone is seen at a much less height; and there can be little doubt
that the yellowish sandy clay, on a level with the river, between the
Carcarana and S. Nicholas, belongs to this same formation; as perhaps
do the beds of sand at Buenos Ayres, which lie at the bottom of the
Pampean formation, about sixty feet beneath the surface of the Plata.
The southerly declination of these beds may perhaps be due, not to
unequal elevation, but to the original form of the bottom of the sea,
sloping from land situated to the north; for that land existed at no
great distance, we have evidence in the vegetable remains in the lowest
bed at St. Fe; and in the silicified wood and in the bones of Toxodon
Paranensis, found (according to M. d’Orbigny) in still lower strata.
BANDA ORIENTAL.
This province lies on the northern side of the Plata, and eastward of
the Uruguay: it has a gentle undulatory surface, with a basis of
primary rocks; and is in most parts covered up with an unstratified
mass, of no great thickness, of reddish Pampean mud. In the eastern
half, near Maldonado, this deposit is more arenaceous than in the
Pampas, it contains many though small concretions of marl or
tosca-rock, and others of highly ferruginous sandstone; in one section,
only a few yards in depth, it rested on stratified sand. Near Monte
Video this deposit in some spots appears to be of greater thickness;
and the remains of the Glyptodon and other extinct mammifers have been
found in it. In the long line of cliffs, between fifty and sixty feet
in height, called the Barrancas de S. Gregorio, which extend westward
of the Rio S. Lucia, the lower half is formed of coarse sand of quartz
and feldspar without mica, like that now cast up on the beach near
Maldonado; and the upper half of Pampean mud, varying in colour and
containing honeycombed veins of soft calcareous matter and small
concretions of tosca-rock arranged in lines, and likewise a few pebbles
of quartz. This deposit fills up hollows and furrows in the underlying
sand; appearing as if water charged with mud had invaded a sandy beach.
These cliffs extend far westward, and at a distance of sixty miles,
near Colonia del Sacramiento, I found the Pampean deposit resting in
some places on this sand, and in others on the primary rocks: between
the sand and the reddish mud, there appeared to be interposed, but the
section was not a very good one, a thin bed of shells of an existing
Mytilus, still partially retaining their colour. The Pampean formation
in Banda Oriental might readily be mistaken for an alluvial deposit:
compared with that of the Pampas, it is often more sandy, and contains
small fragments of quartz; the concretions are much smaller, and there
are no extensive masses of tosca-rock.
In the extreme western parts of this province, between the Uruguay and
a line drawn from Colonia to the R. Perdido (a tributary of the R.
Negro), the formations are far more complicated. Besides primary rocks,
we meet with extensive tracts and many flat-topped, horizontally
stratified, cliff- bounded, isolated hills of tertiary strata, varying
extraordinarily in mineralogical nature, some identical with the old
marine beds of St. Fe Bajada, and some with those of the much more
recent Pampean formation. There are, also, extensive LOW tracts of
country covered with a deposit containing mammiferous remains,
precisely like that just described in the more eastern parts of the
province. Although from the smooth and unbroken state of the country, I
never obtained a section of this latter deposit close to the foot of
the higher tertiary hills, yet I have not the least doubt that it is of
quite subsequent origin; having been deposited after the sea had worn
the tertiary strata into the cliff-bounded hills. This later formation,
which is certainly the equivalent of that of the Pampas, is well seen
in the valleys in the estancia of Berquelo, near Mercedes; it here
consists of reddish earth, full of rounded grains of quartz, and with
some small concretions of tosca-rock arranged in horizontal lines, so
as perfectly to resemble, except in containing a little calcareous
matter, the formation in the eastern parts of Banda Oriental, in Entre
Rios, and at other places: in this estancia the skeleton of a great
Edental quadruped was found. In the valley of the Sarandis, at the
distance of only a few miles, this deposit has a somewhat different
character, being whiter, softer, finer-grained, and full of little
cavities, and consequently of little specific gravity; nor does it
contain any concretions or calcareous matter: I here procured a head,
which when first discovered must have been quite perfect, of the
Toxodon Platensis, another of a Mylodon (This head was at first
considered by Professor Owen (in the “Zoology of the ‘Beagle’s’
Voyage”) as belonging to a distinct genus, namely, Glossotherium.),
perhaps M. Darwinii, and a large piece of dermal armour, differing from
that of the Glyptodon clavipes. These bones are remarkable from their
extraordinarily fresh appearance; when held over a lamp of spirits of
wine, they give out a strong odour and burn with a small flame; Mr. T.
Reeks has been so kind as to analyse some of the fragments, and he
finds that they contain about 7 per cent of animal matter, and 8 per
cent of water. (Liebig “Chemistry of Agriculture” page 194 states that
fresh dry bones contain from 32 to 33 per cent of dry gelatine. See
also Dr. Daubeny, in “Edinburgh New Philosophical Journal” volume 37
page 293.)
The older tertiary strata, forming the higher isolated hills and
extensive tracts of country, vary, as I have said, extraordinarily in
composition: within the distance of a few miles, I sometimes passed
over crystalline limestone with agate, calcareous tuffs, and marly
rocks, all passing into each other,—red and pale mud with concretions
of tosca-rock, quite like the Pampean formation,—calcareous
conglomerates and sandstones,—bright red sandstones passing either into
red conglomerate, or into white sandstone,—hard siliceous sandstones,
jaspery and chalcedonic rocks, and numerous other subordinate
varieties. I was unable to mark out the relations of all these strata,
and will describe only a few distinct sections:—in the cliffs between
P. Gorda on the Uruguay and the A. de Vivoras, the upper bed is
crystalline cellular limestone often passing into calcareous sandstone,
with impressions of some of the same shells as at St. Fe Bajada; at P.
Gorda, this limestone is interstratified with and rests on, white sand,
which covers a bed about thirty feet thick of pale-coloured clay, with
many shells of the great Ostrea Patagonica (In my “Journal” page 171
1st edition, I have hastily and inaccurately stated that the Pampean
mud, which is found over the eastern part of B. Oriental, lies OVER the
limestone at P. Gorda; I should have said that there was reason to
infer that it was a subsequent or superior deposit.): beneath this, in
the vertical cliff, nearly on a level with the river, there is a bed of
red mud absolutely like the Pampean deposit, with numerous often large
concretions of perfectly characterised white, compact tosca-rock. At
the mouth of the Vivoras, the river flows over a pale cavernous
tosca-rock, quite like that in the Pampas, and this APPEARED to
underlie the crystalline limestone; but the section was not unequivocal
like that at P. Gorda. These beds now form only a narrow and much
denuded strip of land; but they must once have extended much further;
for on the next stream, south of the S. Juan, Captain Sulivan, R.N.,
found a little cliff, only just above the surface of the river, with
numerous shells of the Venus Munsterii, D’Orbigny,—one of the species
occurring at St. Fe, and of which there are casts at P. Gorda: the line
of cliffs of the subsequently deposited true Pampean mud, extend from
Colonia to within half a mile of this spot, and no doubt once covered
up this denuded marine stratum. Again at Colonia, a Frenchman found, in
digging the foundations of a house, a great mass of the Ostrea
Patagonica (of which I saw many fragments), packed together just
beneath the surface, and directly superimposed on the gneiss. These
sections are important: M. d’Orbigny is unwilling to believe that beds
of the same nature with the Pampean formation ever underlie the ancient
marine tertiary strata; and I was as much surprised at it as he could
have been; but the vertical cliff at P. Gorda allowed of no mistake,
and I must be permitted to affirm, that after having examined the
country from the Colorado to St. Fe Bajada, I could not be deceived in
the mineralogical character of the Pampean deposit.
Moreover, in a precipitous part of the ravine of Las Bocas, a red
sandstone is distinctly seen to overlie a thick bed of pale mud, also
quite like the Pampean formation, abounding with concretions of true
tosca-rock. This sandstone extends over many miles of country: it is as
red as the brightest volcanic scoriae; it sometimes passes into a
coarse red conglomerate composed of the underlying primary rocks; and
often passes into a soft white sandstone with red streaks. At the
Calera de los Huerfanos, only a quarter of a mile south of where I
first met with the red sandstone, the crystalline white limestone is
quarried: as this bed is the uppermost, and as it often passes into
calcareous sandstone, interstratified with pure sand; and as the red
sandstone likewise passes into soft white sandstone, and is also the
uppermost bed, I believe that these two beds, though so different, are
equivalents. A few leagues southward of these two places, on each side
of the low primary range of S. Juan, there are some flat-topped,
cliff-bounded, separate little hills, very similar to those fringing
the primary ranges in the great plain south of Buenos Ayres: they are
composed—1st, of calcareous tuff with many particles of quartz,
sometimes passing into a coarse conglomerate; 2nd, of a stone
undistinguishable on the closest inspection from the compacter
varieties of tosca-rock; and 3rd, of semi-crystalline limestone,
including nodules of agate: these three varieties pass insensibly into
each other, and as they form the uppermost stratum in this district, I
believe that they, also, are the equivalents of the pure crystalline
limestone, and of the red and white sandstones and conglomerates.
Between these points and Mercedes on the Rio Negro, there are scarcely
any good sections, the road passing over limestone, tosca-rock,
calcareous and bright red sandstones, and near the source of the San
Salvador over a wide extent of jaspery rocks, with much milky agate,
like that in the limestone near San Juan. In the estancia of Berquelo,
the separate, flat-topped, cliff-bounded hills are rather higher than
in the other parts of the country; they range in a N.E. and S.W.
direction; their uppermost beds consist of the same bright red
sandstone, passing sometimes into a conglomerate, and in the lower part
into soft white sandstone, and even into loose sand: beneath this
sandstone, I saw in two places layers of calcareous and marly rocks,
and in one place red Pampean-like earth; at the base of these sections,
there was a hard, stratified, white sandstone, with chalcedonic layers.
Near Mercedes, beds of the same nature and apparently of the same age,
are associated with compact, white, crystalline limestone, including
much botryoidal agate, and singular masses, like porcelain, but really
composed of a calcareo-siliceous paste. In sinking wells in this
district the chalcedonic strata seem to be the lowest. Beds, such as
there described, occur over the whole of this neighbourhood; but twenty
miles further up the R. Negro, in the cliffs of Perika, which are about
fifty feet in height, the upper bed is a prettily variegated
chalcedony, mingled with a pure white tallowy limestone; beneath this
there is a conglomerate of quartz and granite; beneath this many
sandstones, some highly calcareous; and the whole lower two-thirds of
the cliff consists of earthy calcareous beds of various degrees of
purity, with one layer of reddish Pampean-like mud.
When examining the agates, the chalcedonic and jaspery rocks, some of
the limestones, and even the bright red sandstones, I was forcibly
struck with their resemblance to deposits formed in the neighbourhood
of volcanic action. I now find that M. Isabelle, in his “Voyage a
Buenos Ayres,” has described closely similar beds on Itaquy and Ibicuy
(which enter the Uruguay some way north of the R. Negro) and these beds
include fragments of red decomposed true scoriae hardened by zeolite,
and of black retinite: we have then here good evidence of volcanic
action during our tertiary period. Still further north, near S. Anna,
where the Parana makes a remarkable bend, M. Bonpland found some
singular amygdaloidal rocks, which perhaps may belong to this same
epoch. (M. d’Orbigny “Voyage” Part. Geolog. page 29) I may remark that,
judging from the size and well-rounded condition of the blocks of rock
in the above-described conglomerates, masses of primary formation
probably existed at this tertiary period above water: there is, also,
according to M. Isabelle, much conglomerate further north, at Salto.
From whatever source and through whatever means the great Pampean
formation originated, we here have, I must repeat, unequivocal evidence
of a similar action at a period before that of the deposition of the
marine tertiary strata with extinct shells, at Santa Fe and P. Gorda.
During also the deposition of these strata, we have in the intercalated
layers of red Pampean-like mud and tosca-rock, and in the passage near
S. Juan of the semi-crystalline limestones with agate into tosca
undistinguishable from that of the Pampas, evidence of the same action,
though continued only at intervals and in a feeble manner. We have
further seen that in this district, at a period not only subsequent to
the deposition of the tertiary strata, but to their upheavement and
most extensive denudation, true Pampean mud with its usual characters
and including mammiferous remains, was deposited round and between the
hills or islets formed of these tertiary strata, and over the whole
eastern and low primary districts of Banda Oriental.
EARTHY MASS, WITH EXTINCT MAMMIFEROUS REMAINS, OVER THE PORPHYRITIC
GRAVEL AT S. JULIAN, LATITUDE 49 DEGREES 14′ S., IN PATAGONIA.
(FIGURE 16. SECTION OF THE LOWEST PLAIN AT PORT S. JULIAN.
(Section through beds from top to bottom: A, B, C, D, E, F.)
AA. Superficial bed of reddish earth, with the remains of the
Macrauchenia, and with recent sea-shells on the surface.
B. Gravel of porphyritic rocks.
C. and D. Pumiceous mudstone.—Ancient tertiary formation.
E. and F. Sandstone and argillaceous beds.—Ancient tertiary formation.)
This case, though not coming strictly under the Pampean formation, may
be conveniently given here. On the south side of the harbour, there is
a nearly level plain (mentioned in the First Chapter) about seven miles
long, and three or four miles wide, estimated at ninety feet in height,
and bordered by perpendicular cliffs, of which a section is represented
in Figure 16.
The lower old tertiary strata (to be described in the next chapter) are
covered by the usual gravel bed; and this by an irregular earthy,
sometimes sandy mass, seldom more than two or three feet in thickness,
except where it fills up furrows or gullies worn not only through the
underlying gravel, but even through the upper tertiary beds. This
earthy mass is of a pale reddish colour, like the less pure varieties
of Pampean mud in Banda Oriental; it includes small calcareous
concretions, like those of tosca- rock but more arenaceous, and other
concretions of a greenish, indurated argillaceous substance: a few
pebbles, also, from the underlying gravel-bed are also included in it,
and these being occasionally arranged in horizontal lines, show that
the mass is of sub-aqueous origin. On the surface and embedded in the
superficial parts, there are numerous shells, partially retaining their
colours, of three or four of the now commonest littoral species. Near
the bottom of one deep furrow (represented in Figure 16), filled up
with this earthy deposit, I found a large part of the skeleton of the
Macrauchenia Patachonica—a gigantic and most extraordinary pachyderm,
allied, according to Professor Owen, to the Palaeotherium, but with
affinities to the Ruminants, especially to the American division of the
Camelidae. Several of the vertebrae in a chain, and nearly all the
bones of one of the limbs, even to the smallest bones of the foot, were
embedded in their proper relative positions: hence the skeleton was
certainly united by its flesh or ligaments, when enveloped in the mud.
This earthy mass, with its concretions and mammiferous remains, filling
up furrows in the underlying gravel, certainly presents a very striking
resemblance to some of the sections (for instance, at P. Alta in B.
Blanca, or at the Barrancas de S. Gregorio) in the Pampean formation;
but I must believe that this resemblance is only accidental. I suspect
that the mud which at the present day is accumulating in deep and
narrow gullies at the head of the harbour, would, after elevation,
present a very similar appearance. The southernmost part of the true
Pampean formation, namely, on the Colorado, lies 560 miles of latitude
north of this point. (In the succeeding chapter I shall have to refer
to a great deposit of extinct mammiferous remains, lately discovered by
Captain Sulivan, R.N., at a point still further south, namely, at the
R. Gallegos; their age must at present remain doubtful.)
With respect to the age of the Macrauchenia, the shells on the surface
prove that the mass in which the skeleton was enveloped has been
elevated above the sea within the recent period: I did not see any of
the shells embedded at a sufficient depth to assure me (though it be
highly probable) that the whole thickness of the mass was
contemporaneous with these INDIVIDUAL SPECIMENS. That the Macrauchenia
lived subsequently to the spreading out of the gravel on this plain is
certain; and that this gravel, at the height of ninety feet, was spread
out long after the existence of recent shells, is scarcely less
certain. For, it was shown in the First Chapter, that this line of
coast has been upheaved with remarkable equability, and that over a
vast space both north and south of S. Julian, recent species of shells
are strewed on (or embedded in) the surface of the 250 feet plain, and
of the 350 feet plain up to a height of 400 feet. These wide
step-formed plains have been formed by the denuding action of the
coast-waves on the old tertiary strata; and therefore, when the surface
of the 350 feet plain, with the shells on it, first rose above the
level of the sea, the 250 feet plain did not exist, and its formation,
as well as the spreading out of the gravel on its summit, must have
taken place subsequently. So also the denudation and the
gravel-covering of the 90 feet plain must have taken place subsequently
to the elevation of the 250 feet plain, on which recent shells are also
strewed. Hence there cannot be any doubt that the Macrauchenia, which
certainly was entombed in a fresh state, and which must have been alive
after the spreading out of the gravel on the 90 feet plain, existed,
not only subsequently to the upraised shells on the surface of the 250
feet plain, but also to those on the 350 to 400 feet plain: these
shells, eight in number (namely, three species of Mytilus, two of
Patella, one Fusus, Voluta, and Balanus), are undoubtedly recent
species, and are the commonest kinds now living on this coast. At Punta
Alta in B. Blanca, I remarked how marvellous it was, that the Toxodon,
a mammifer so unlike to all known genera, should have co-existed with
twenty- three still living marine animals; and now we find that the
Macrauchenia, a quadruped only a little less anomalous than the
Toxodon, also co-existed with eight other still existing Mollusca: it
should, moreover, be borne in mind, that a tooth of a pachydermatous
animal was found with the other remains at Punta Alta, which Professor
Owen thinks almost certainly belonged to the Macrauchenia.
Mr. Lyell has arrived at a highly important conclusion with respect to
the age of the North American extinct mammifers (many of which are
closely allied to, and even identical with, those of the Pampean
formation), namely, that they lived subsequently to the period when
erratic boulders were transported by the agency of floating ice in
temperate latitudes. (“Geological Proceedings” volume 4 page 36.) Now
in the valley of the Santa Cruz, only fifty miles of latitude south of
the spot where the Macrauchenia was entombed, vast numbers of gigantic,
angular boulders, which must have been transported from the Cordillera
on icebergs, lie strewed on the plain, at the height of 1,400 feet
above the level of the sea. In ascending to this level, several
step-formed plains must be crossed, all of which have necessarily
required long time for their formation; hence the lowest or ninety feet
plain, with its superficial bed containing the remains of the
Macrauchenia, must have been formed very long subsequently to the
period when the 1,400 feet plain was beneath the sea, and boulders were
dropped on it from floating masses of ice. (It must not be inferred
from these remarks, that the ice-action ceased in South America at this
comparatively ancient period; for in Tierra del Fuego boulders were
probably transported contemporaneously with, if not subsequently to,
the formation of the ninety feet plain at S. Julian, and at other parts
of the coast of Patagonia.) Mr. Lyell’s conclusion, therefore, is thus
far confirmed in the southern hemisphere; and it is the more important,
as one is naturally tempted to admit so simple an explanation, that it
was the ice-period that caused the extinction of the numerous great
mammifers which so lately swarmed over the two Americas.
A SUMMARY AND CONCLUDING REMARKS ON THE PAMPEAN FORMATION.
One of its most striking features is its great extent; I passed
continuously over it from the Colorado to St. Fe Bajada, a distance of
500 geographical miles; and M. d’Orbigny traced it for 250 miles
further north. In the latitude of the Plata, I examined this formation
at intervals over an east and west line of 300 miles from Maldonado to
the R. Carcarana; and M. d’Orbigny believes it extends 100 miles
further inland: from Mr. Caldcleugh’s travels, however, I should have
thought that it had extended, south of the Cordovese range, to near
Mendoza, and I may add that I heard of great bones having been found
high up the R. Quinto. Hence the area of the Pampean formation, as
remarked by M. d’Orbigny, is probably at least equal to that of France,
and perhaps twice or thrice as great. In a basin, surrounded by
gravel-cliff (at a height of nearly three thousand feet), south of
Mendoza, there is, as described in the Third Chapter, a deposit very
like the Pampean, interstratified with other matter; and again at S.
Julian’s, in Patagonia, 560 miles south of the Colorado, a small
irregular bed of a nearly similar nature contains, as we have just
seen, mammiferous remains. In the provinces of Moxos and Chiquitos
(1,000 miles northward of the Pampas), and in Bolivia, at a height of
4,000 metres, M. d’Orbigny has described similar deposits, which he
believes to have been formed by the same agency contemporaneously with
the Pampean formation. Considering the immense distances between these
several points, and their different heights, it appears to me
infinitely more probable, that this similarity has resulted not from
contemporaneousness of origin, but from the similarity of the rocky
framework of the continent: it is known that in Brazil an immense area
consists of gneissic rocks, and we shall hereafter see, over how great
a length the plutonic rocks of the Cordillera, the overlying purple
porphyries, and the trachytic ejections, are almost identical in
nature.
Three theories on the origin of the Pampean formation have been
propounded:—First, that of a great debacle by M. d’Orbigny; this seems
founded chiefly on the absence of stratification, and on the number of
embedded remains of terrestrial quadrupeds. Although the Pampean
formation (like so many argillaceous deposits) is not divided into
distinct and separate strata, yet we have seen that in one good section
it was striped with horizontal zones of colour, and that in several
specified places the upper and lower parts differed, not only
considerably in colour, but greatly in constitution. In the southern
part of the Pampas the upper mass (to a certain extent stratified)
generally consists of hard tosca-rock, and the lower part of red
Pampean mud, often itself divided into two or more masses, varying in
colour and in the quantity of included calcareous matter. In Western
Banda Oriental, beds of a similar nature, but of a greater age,
conformably underlie and are intercalated with the regularly stratified
tertiary formation. As a general rule, the marly concretions are
arranged in horizontal lines, sometimes united into irregular strata:
surely, if the mud had been tumultuously deposited in mass, the
included calcareous matter would have segregated itself irregularly,
and not into nodules arranged in horizontal lines, one above the other
and often far apart: this arrangement appears to me to prove that mud,
differing slightly in composition, was successively and quietly
deposited. On the theory of a debacle, a prodigious amount of mud,
without a single pebble, is supposed to have been borne over the wide
surface of the Pampas, when under water: on the other hand, over the
whole of Patagonia, the same or another debacle is supposed to have
borne nothing but gravel,—the gravel and the fine mud in the
neighbourhood of the Rios Negro and Colorado having been borne to an
equal distance from the Cordillera, or imagined line of disturbance:
assuredly directly opposite effects ought not to be attributed to the
same agency. Where, again, could a mass of fine sediment, charged with
calcareous matter in a fit state for chemical segregation, and in
quantity sufficient to cover an area at least 750 miles long, and 400
miles broad, to a depth of from twenty to thirty feet to a hundred
feet, have been accumulated, ready to be transported by the supposed
debacle? To my mind it is little short of demonstration, that a great
lapse of time was necessary for the production and deposition of the
enormous amount of mudlike matter forming the Pampas; nor should I have
noticed the theory of a debacle, had it not been adduced by a
naturalist so eminent as M. d’Orbigny.
A second theory, first suggested, I believe, by Sir W. Parish, is that
the Pampean formation was thrown down on low and marshy plains by the
rivers of this country before they assumed their present courses. The
appearance and composition of the deposit, the manner in which it
slopes up and round the primary ranges, the nature of the underlying
marine beds, the estuary and sea-shells on the surface, the overlying
sandstone beds at M. Hermoso, are all quite opposed to this view. Nor
do I believe that there is a single instance of a skeleton of one of
the extinct mammifers having been found in an upright position, as if
it had been mired.
The third theory, of the truth of which I cannot entertain the smallest
doubt, is that the Pampean formation was slowly accumulated at the
mouth of the former estuary of the Plata and in the sea adjoining it. I
have come to this conclusion from the reasons assigned against the two
foregoing theories, and from simple geographical considerations. From
the numerous shells of the Azara labiata lying loose on the surface of
the plains, and near Buenos Ayres embedded in the tosca-rock, we know
that this formation not only was formerly covered by, but that the
uppermost parts were deposited in, the brackish water of the ancient La
Plata. Southward and seaward of Buenos Ayres, the plains were upheaved
from under water inhabited by true marine shells. We further know from
Professor Ehrenberg’s examination of the twenty microscopical organisms
in the mud round the tooth of the Mastodon high up the course of the
Parana, that the bottom- most part of this formation was of
brackish-water origin. A similar conclusion must be extended to the
beds of like composition, at the level of the sea and under it, at M.
Hermoso in Bahia Blanca. Dr. Carpenter finds that the harder varieties
of tosca-rock, collected chiefly to the south, contain marine spongoid
bodies, minute fragments of shells, corals, and Polythalamia; these
perhaps may have been drifted inwards by the tides, from the more open
parts of the sea. The absence of shells, throughout this deposit, with
the exception of the uppermost layers near Buenos Ayres, is a
remarkable fact: can it be explained by the brackish condition of the
water, or by the deep mud at the bottom? I have stated that both the
reddish mud and the concretions of tosca-rock are often penetrated by
minute, linear cavities, such as frequently may be observed in
fresh-water calcareous deposits:—were they produced by the burrowing of
small worms? Only on this view of the Pampean formation having been of
estuary origin, can the extraordinary numbers (presently to be alluded
to) of the embedded mammiferous remains be explained. (It is almost
superfluous to give the numerous cases (for instance, in Sumatra; Lyell
“Principles” volume 3 page 325 sixth edition, of the carcasses of
animals having been washed out to sea by swollen rivers; but I may
refer to a recent account by Mr. Bettington “Asiatic Society” 1845 June
21st, of oxen, deer, and bears being carried into the Gulf of Cambray;
see also the account in my “Journal” 2nd edition page 133, of the
numbers of animals drowned in the Plata during the great, often
recurrent, droughts.)
With respect to the first origin of the reddish mud, I will only
remark, that the enormous area of Brazil consists in chief part of
gneissic and other granitic rocks, which have suffered decomposition,
and been converted into a red, gritty, argillaceous mass, to a greater
depth than in any other country which I have seen. The mixture of
rounded grains, and even of small fragments and pebbles of quartz, in
the Pampean mud of Banda Oriental, is evidently due to the neighbouring
and underlying primary rocks. The estuary mud was drifted during the
Pampean period in a much more southerly course, owing probably to the
east and west primary ridges south of the Plata not having been then
elevated, than the mud of the Plata at present is; for it was formerly
deposited as far south as the Colorado. The quantity of calcareous
matter in this formation, especially in those large districts where the
whole mass passes into tosca-rock, is very great: I have already
remarked on the close resemblance in external and microscopical
appearance, between this tosca-rock and the strata at Coquimbo, which
have certainly resulted from the decay and attrition of recent shells:
I dare not, however, extend this conclusion to the calcareous rocks of
the Pampas, more especially as the underlying tertiary strata in
western Banda Oriental show that at that period there was a copious
emission of carbonate of lime, in connection with volcanic action. (I
may add, that there are nearly similar superficial calcareous beds at
King George’s Sound in Australia; and these undoubtedly have been
formed by the disintegration of marine remains see “Volcanic Islands”
etc. page 144. There is, however, something very remarkable in the
frequency of superficial, thin beds of earthy calcareous matter, in
districts where the surrounding rocks are not calcareous. Major
Charters, in a Paper read before the Geographical Society April 13,
1840 and abstracted in the “Athenaeum” page 317, states that this is
the case in parts of Mexico, and that he has observed similar
appearances in many parts of South Africa. The circumstance of the
uppermost stratum round the ragged Sierra Ventana, consisting of
calcareous or marly matter, without any covering of alluvial matter,
strikes me as very singular, in whatever manner we view the deposition
and elevation of the Pampean formation.)
The Pampean formation, judging from its similar composition, and from
the apparent absolute specific identity of some of its mammiferous
remains, and from the generic resemblance of others, belongs over its
vast area— throughout Banda Oriental, Entre Rios, and the wide extent
of the Pampas as far south as the Colorado,—to the same geological
epoch. The mammiferous remains occur at all depths from the top to the
bottom of the deposit; and I may add that nowhere in the Pampas is
there any appearance of much superficial denudation: some bones which I
found near the Guardia del Monte were embedded close to the surface;
and this appears to have been the case with many of those discovered in
Banda Oriental: on the Matanzas, twenty miles south of Buenos Ayres, a
Glyptodon was embedded five feet beneath the surface; numerous remains
were found by S. Muniz, near Luxan, at an average depth of eighteen
feet; in Buenos Ayres a skeleton was disinterred at sixty feet depth,
and on the Parana I have described two skeletons of the Mastodon only
five or six feet above the very base of the deposit. With respect to
the age of this formation, as judged of by the ordinary standard of the
existence of Mollusca, the only evidence within the limits of the true
Pampas which is at all trustworthy, is afforded by the still living
Azara labiata being embedded in tosca-rock near Buenos Ayres. At Punta
Alta, however, we have seen that several of the extinct mammifers, most
characteristic of the Pampean formation, co-existed with twenty species
of Mollusca, a barnacle and two corals, all still living on this same
coast;— for when we remember that the shells have a more ancient
appearance than the bones; that many of the bones, though embedded in a
coarse conglomerate, are perfectly preserved; that almost all the parts
of the skeleton of the Scelidotherium, even to the knee-cap, were lying
in their proper relative positions; and that a large piece of the
fragile dermal armour of a Dasypoid quadruped, connected with some of
the bones of the foot, had been entombed in a condition allowing the
two sides to be doubled together, it must assuredly be admitted that
these mammiferous remains were embedded in a fresh state, and therefore
that the living animals co-existed with the co-embedded shells.
Moreover, the Macrauchenia Patachonica (of which, according to
Professor Owen, remains also occur in the Pampas of Buenos Ayres, and
at Punta Alta) has been shown by satisfactory evidence of another kind,
to have lived on the plains of Patagonia long after the period when the
adjoining sea was first tenanted by its present commonest molluscous
animals. We must, therefore, conclude that the Pampean formation
belongs, in the ordinary geological sense of the word, to the Recent
Period. (M. d’Orbigny believes “Voyage” Part. Geolog. page 81, that
this formation, though “tres voisine de la notre, est neanmoins de
beaucoup anterieure a notre creation.”)
At St. Fe Bajada, the Pampean estuary formation, with its mammiferous
remains, conformably overlies the marine tertiary strata, which (as
first shown by M. d’Orbigny) are contemporaneous with those of
Patagonia, and which, as we shall hereafter see, belong to a very
ancient tertiary stage. When examining the junction between these two
formations, I thought that the concretionary layer of marl marked a
passage between the marine and estuary stages. M. d’Orbigny disputes
this view (as given in my “Journal”), and I admit that it is erroneous,
though in some degree excusable, from their conformability and from
both abounding with calcareous matter. It would, indeed, have been a
great anomaly if there had been a true passage between a deposit
contemporaneous with existing species of mollusca, and one in which all
the mollusca appear to be extinct. Northward of Santa Fe, M. d’Orbigny
met with ferruginous sandstones, marly rocks, and other beds, which he
considers as a distinct and lower formation; but the evidence that they
are not parts of the same with an altered mineralogical character, does
not appear to me quite satisfactory.
In Western Banda Oriental, while the marine tertiary strata were
accumulating, there were volcanic eruptions, much silex and lime were
precipitated from solution, coarse conglomerates were formed, being
derived probably from adjoining land, and layers of red mud and marly
rocks, like those of the Pampean formation, were occasionally
deposited. The true Pampean deposit, with mammiferous remains, instead
of as at Santa Fe overlying conformably the tertiary strata, is here
seen at a lower level folding round and between the flat-topped,
cliff-bounded hills, formed by a upheaval and denudation of these same
tertiary strata. The upheaval, having occurred here earlier than at
Santa Fe, may be naturally accounted for by the contemporaneous
volcanic action. At the Barrancas de S. Gregorio, the Pampean deposit,
as we have seen, overlies and fills up furrows in coarse sand,
precisely like that now accumulating on the shores near the mouth of
the Plata. I can hardly believe that this loose and coarse sand is
contemporaneous with the old tertiary and often crystalline strata of
the more western parts of the province; and am induced to suspect that
it is of subsequent origin. If that section near Colonia could be
implicitly trusted, in which, at a height of only fifteen feet above
the Plata, a bed of fresh-looking mussels, of an existing littoral
species, appeared to lie between the sand and the Pampean mud, I should
conclude that Banda Oriental must have stood, when the coarse sand was
accumulating, at only a little below its present level, and had then
subsided, allowing the estuary Pampean mud to cover far and wide its
surface up to a height of some hundred feet; and that after this
subsidence the province had been uplifted to its present level.
In Western Banda Oriental, we know, from two unequivocal sections that
there is a mass, absolutely undistinguishable from the true Pampean
deposit, beneath the old tertiary strata. This inferior mass must be
very much more ancient than the upper deposit with its mammiferous
remains, for it lies beneath the tertiary strata in which all the
shells are extinct. Nevertheless, the lower and upper masses, as well
as some intermediate layers, are so similar in mineralogical character,
that I cannot doubt that they are all of estuary origin, and have been
derived from the same great source. At first it appeared to me
extremely improbable, that mud of the same nature should have been
deposited on nearly the same spot, during an immense lapse of time,
namely, from a period equivalent perhaps to the Eocene of Europe to
that of the Pampean formation. But as, at the very commencement of the
Pampean period, if not at a still earlier period, the Sierra Ventana
formed a boundary to the south,—the Cordillera or the plains in front
of them to the west,—the whole province of Corrientes probably to the
north, for, according to M. d’Orbigny, it is not covered by the Pampean
deposit,—and Brazil, as known by the remains in the caves, to the
north-east; and as again, during the older tertiary period, land
already existed in Western Banda Oriental and near St. Fe Bajada, as
may be inferred from the vegetable debris, from the quantities of
silicified wood, and from the remains of a Toxodon found, according to
M. d’Orbigny, in still lower strata, we may conclude, that at this
ancient period a great expanse of water was surrounded by the same
rocky framework which now bounds the plains of Pampean formation. This
having been the case, the circumstance of sediment of the same nature
having been deposited in the same area during an immense lapse of time,
though highly remarkable, does not appear incredible.
The elevation of the Pampas, at least of the southern parts, has been
slow and interrupted by several periods of rest, as may be inferred
from the plains, cliffs, and lines of sand-dunes (with shells and
pumice-pebbles) standing at different heights. I believe, also, that
the Pampean mud continued to be deposited, after parts of this
formation had already been elevated, in the same manner as mud would
continue to be deposited in the estuary of the Plata, if the mud-banks
on its shores were now uplifted and changed into plains: I believe in
this from the improbability of so many skeletons and bones having been
accumulated at one spot, where M. Hermoso now stands, at a depth of
between eight hundred and one thousand feet, and at a vast distance
from any land except small rocky islets,—as must have been the case, if
the high tosca-plain round the Ventana and adjoining Sierras, had not
been already uplifted and converted into land, supporting mammiferous
animals. At Punta Alta we have good evidence that the gravel- strata,
which certainly belong to the true Pampean period, were accumulated
after the elevation in that neighbourhood of the main part of the
Pampean deposit, whence the rounded masses of tosca-rock were derived,
and that rolled fragment of black bone in the same peculiar condition
with the remains at Monte Hermoso.
The number of the mammiferous remains embedded in the Pampas is, as I
have remarked, wonderful: it should be borne in mind that they have
almost exclusively been found in the cliffs and steep banks of rivers,
and that, until lately, they excited no attention amongst the
inhabitants: I am firmly convinced that a deep trench could not be cut
in any line across the Pampas, without intersecting the remains of some
quadruped. It is difficult to form an opinion in what part of the
Pampas they are most numerous; in a limited spot they could not well
have been more numerous than they were at P. Alta; the number, however,
lately found by Senor F. Muniz, near Luxan, in a central spot in the
Pampas, is extraordinarily great: at the end of this chapter I will
give a list of all the localities at which I have heard of remains
having been discovered. Very frequently the remains consist of almost
perfect skeletons; but there are, also, numerous single bones, as for
instance at St. Fe. Their state of preservation varies much, even when
embedded near each other: I saw none others so perfectly preserved as
the heads of the Toxodon and Mylodon from the white soft earthy bed on
the Sarandis in Banda Oriental. It is remarkable that in two limited
sections I found no less than five teeth separately embedded, and I
heard of teeth having been similarly found in other parts: may we
suppose that the skeletons or heads were for a long time gently drifted
by currents over the soft muddy bottom, and that the teeth
occasionally, here and there, dropped out?
It may be naturally asked, where did these numerous animals live? From
the remarkable discoveries of MM. Lund and Clausen, it appears that
some of the species found in the Pampas inhabited the highlands of
Brazil: the Mastodon Andium is embedded at great heights in the
Cordillera from north of the equator to at least as far south as Tarija
(Humboldt states that the Mastodon has been discovered in New Granada:
it has been found in Quito. When at Lima, I saw a tooth of a Mastodon
in the possession of Don M. Rivero, found at Playa Chica on the
Maranon, near the Guallaga. Every one has heard of the numerous remains
of Mastodon in Bolivia.); and as there is no higher land, there can be
little doubt that this Mastodon must have lived on the plains and
valleys of that great range. These countries, however, appear too far
distant for the habitation of the individuals entombed in the Pampas:
we must probably look to nearer points, for instance to the province of
Corrientes, which, as already remarked, is said not to be covered by
the Pampean formation, and may therefore, at the period of its
deposition, have existed as dry land. I have already given my reasons
for believing that the animals embedded at M. Hermoso and at P. Alta in
Bahia Blanca, lived on adjoining land, formed of parts of the already
elevated Pampean deposit. With respect to the food of these many great
extinct quadrupeds, I will not repeat the facts given in my “Journal”
(second edition page 85), showing that there is no correlation between
the luxuriance of the vegetation of a country and the size of its
mammiferous inhabitants. I do not doubt that large animals could now
exist, as far as the amount, not kind, of vegetation is concerned, on
the sterile plains of Bahia Blanca and of the R. Negro, as well as on
the equally, if not more sterile plains of Southern Africa. The
climate, however, may perhaps have somewhat deteriorated since the
mammifers embedded at Bahia Blanca lived there; for we must not infer,
from the continued existence of the same shells on the present coasts,
that there has been no change in climate; for several of these shells
now range northward along the shores of Brazil, where the most
luxuriant vegetation flourishes under a tropical temperature. With
respect to the extinction, which at first fills the mind with
astonishment, of the many great and small mammifers of this period, I
may also refer to the work above cited (second edition page 173), in
which I have endeavoured to show, that however unable we may be to
explain the precise cause, we ought not properly to feel more surprised
at a species becoming extinct than at one being rare; and yet we are
accustomed to view the rarity of any particular species as an ordinary
event, not requiring any extraordinary agency.
The several mammifers embedded in the Pampean formation, which mostly
belong to extinct genera, and some even to extinct families or orders,
and which differ nearly, if not quite, as much as do the Eocene
mammifers of Europe from living quadrupeds having existed
contemporaneously with mollusca, all still inhabiting the adjoining
sea, is certainly a most striking fact. It is, however, far from being
an isolated one; for, during the late tertiary deposits of Britain, an
elephant, rhinoceros, and hippopotamus co-existed with many recent land
and fresh-water shells; and in North America, we have the best evidence
that a mastodon, elephant, megatherium, megalonyx, mylodon, an extinct
horse and ox, likewise co- existed with numerous land, fresh-water, and
marine recent shells. (Many original observations, and a summary on
this subject, are given in Mr. Lyell’s paper in the “Geological
Proceedings” volume 4 page 3 and in his “Travels in North America”
volume 1 page 164 and volume 2 page 60. For the European analogous
cases see Mr. Lyell’s “Principles of Geology” 6th edition volume 1 page
37.) The enumeration of these extinct North American animals naturally
leads me to refer to the former closer relation of the mammiferous
inhabitants of the two Americas, which I have discussed in my
“Journal,” and likewise to the vast extent of country over which some
of them ranged: thus the same species of the Megatherium, Megalonyx,
Equus (as far as the state of their remains permits of identification),
extended from the Southern United States of North America to Bahia
Blanca, in latitude 39 degrees S., on the coast of Patagonia. The fact
of these animals having inhabited tropical and temperate regions, does
not appear to me any great difficulty, seeing that at the Cape of Good
Hope several quadrupeds, such as the elephant and hippopotamus, range
from the equator to latitude 35 degrees south. The case of the Mastodon
Andium is one of more difficulty, for it is found from latitude 36
degrees S., over, as I have reason to believe, nearly the whole of
Brazil, and up the Cordillera to regions which, according to M.
d’Orbigny, border on perpetual snow, and which are almost destitute of
vegetation: undoubtedly the climate of the Cordillera must have been
different when the mastodon inhabited it; but we should not forget the
case of the Siberian mammoth and rhinoceros, as showing how severe a
climate the larger pachydermata can endure; nor overlook the fact of
the guanaco ranging at the present day over the hot low deserts of
Peru, the lofty pinnacles of the Cordillera, and the damp forest-clad
land of Southern Tierra del Fuego; the puma, also, is found from the
equator to the Strait of Magellan, and I have seen its footsteps only a
little below the limits of perpetual snow in the Cordillera of Chile.
At the period, so recent in a geological sense, when these extinct
mammifers existed, the two Americas must have swarmed with quadrupeds,
many of them of gigantic size; for, besides those more particularly
referred to in this chapter, we must include in this same period those
wonderfully numerous remains, some few of them specifically, and others
generically related to those of the Pampas, discovered by MM. Lund and
Clausen in the caves of Brazil. Finally, the facts here given show how
cautious we ought to be in judging of the antiquity of a formation from
even a great amount of difference between the extinct and living
species in any one class of animals;—we ought even to be cautious in
accepting the general proposition, that change in organic forms and
lapse of time are at all, necessarily, correlatives.
LOCALITIES WITHIN THE REGION OF THE PAMPAS WHERE GREAT BONES HAVE BEEN
FOUND.
The following list, which includes every account which I have hitherto
met with of the discovery of fossil mammiferous remains in the Pampas,
may be hereafter useful to a geologist investigating this region, and
it tends to show their extraordinary abundance. I heard of and saw many
fossils, the original position of which I could not ascertain; and I
received many statements too vague to be here inserted. Beginning to
the south:—we have the two stations in Bahia Blanca, described in this
chapter, where at P. Alta, the Megatherium, Megalonyx, Scelidotherium,
Mylodon, Holophractus (or an allied genus), Toxodon, Macrauchenia, and
an Equus were collected; and at M. Hermoso a Ctenomys, Hydrochaerus,
some other rodents and the bones of a great megatheroid quadruped.
Close north-east of the S. Tapalguen, we have the Rios ‘Huesos’ (i.e.
“bones”), which probably takes its name from large fossil bones. Near
Villa Nuevo, and at Las Averias, not far from the Salado, three nearly
perfect skeletons, one of the Megatherium, one of the Glyptodon
clavipes, and one of some great Dasypoid quadruped, were found by the
agent of Sir W. Parish (see his work “Buenos Ayres” etc. page 171). I
have seen the tooth of a Mastodon from the Salado; a little northward
of this river, on the borders of a lake near the G. del Monte, I saw
many bones, and one large piece of dermal armour; higher up the Salado,
there is a place called Monte “Huesos.” On the Matanzas, about twenty
miles south of Buenos Ayres, the skeleton (vide page 178 of “Buenos
Ayres” etc. by Sir W. Parish) of a Glyptodon was found about five feet
beneath the surface; here also (see Catalogue of Royal College of
Surgeons) remains of Glyptodon clavipes, G. ornatus, and G. reticulatus
were found. Signor Angelis, in a letter which I have seen, refers to
some great remains found in Buenos Ayres, at a depth of twenty varas
from the surface. Seven leagues north of this city the same author
found the skeletons of Mylodon robustus and Glyptodon ornatus. From
this neighbourhood he has lately sent to the British Museum the
following fossils:—Remains of three or four individuals of Megatherium;
of three species of Glyptodon; of three individuals of the Mastodon
Andium; of Macrauchenia; of a second species of Toxodon, different from
T. Platensis; and lastly, of the Machairodus, a wonderful large
carnivorous animal. M. d’Orbigny has lately received from the Recolate
“Voyage” Pal. page 144), near Buenos Ayres, a tooth of Toxodon
Platensis.
Proceeding northward, along the west bank of the Parana, we come to the
Rio Luxan, where two skeletons of the Megatherium have been found; and
lately, within eight leagues of the town of Luxan, Dr. F. X. Muniz has
collected (“British Packet” Buenos Ayres September 25, 1841), from an
average depth of eighteen feet, very numerous remains, of no less than,
as he believes, nine distinct species of mammifers. At Areco, large
bones have been found, which are believed, by the inhabitants, to have
been changed from small bones, by the water of the river! At Arrecifes,
the Glyptodon, sent to the College of Surgeons, was found; and I have
seen two teeth of a Mastodon from this quarter. At S. Nicolas, M.
d’Orbigny found remains of a Canis, Ctenomys, and Kerodon; and M.
Isabelle (“Voyage” page 332) refers to a gigantic Armadillo found
there. At S. Carlos, I heard of great bones. A little below the mouth
of the Carcarana, the two skeletons of Mastodon were found; on the
banks of this river, near S. Miguel, I found teeth of the Mastodon and
Toxodon; and “Falkner” (page 55) describes the osseous armour of some
great animal; I heard of many other bones in this neighbourhood. I have
seen, I may add, in the possession of Mr. Caldcleugh, the tooth of a
Mastodon Andium, said to have been found in Paraguay; I may here also
refer to a statement in this gentleman’s travels (volume 1 page 48), of
a great skeleton having been found in the province of Bolivia in
Brazil, on the R. de las Contas. The furthest point westward in the
Pampas, at which I have HEARD of fossil bones, was high up on the banks
of R. Quinto.
In Entre Rios, besides the remains of the Mastodon, Toxodon, Equus, and
a great Dasypoid quadruped near St. Fe Bajada, I received an account of
bones having been found a little S.E. of P. Gorda (on the Parana), and
of an entire skeleton at Matanzas, on the Arroyo del Animal.
In Banda Oriental, besides the remains of the Toxodon, Mylodon, and two
skeletons of great animals with osseous armour (distinct from that of
the Glyptodon), found on the Arroyos Sarandis and Berquelo, M. Isabelle
(“Voyage” page 322) says, many bones have been found near the R. Negro,
and on the R. Arapey, an affluent of the Paraguay, in latitude 30
degrees 40 minutes south. I heard of bones near the source of the A.
Vivoras. I saw the remains of a Dasypoid quadruped from the Arroyo
Seco, close to M. Video; and M. d’Orbigny refers (“Voyage” Geolog. page
24), to another found on the Pedernal, an affluent of the St. Lucia;
and Signor Angelis, in a letter, states that a third skeleton of this
family has been found, near Canelones. I saw a tooth of the Mastodon
from Talas, another affluent of the St. Lucia. The most eastern point
at which I heard of great bones having been found, was at Solis Grande,
between M. Video and Maldonado.
CHAPTER V.
ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.
Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with
Infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P.
Gallegos.—Eastern Tierra del Fuego; leaves of extinct
beech-trees.—Summary on the Patagonian tertiary formations.—Tertiary
formations of the Western Coast.—Chonos and Chiloe groups, volcanic
rocks of.—Concepcion.—Navidad.—Coquimbo.—Summary.—Age of the tertiary
formations.—Lines of elevation.—Silicified wood.—Comparative ranges of
the extinct and living mollusca on the West Coast of—S.
America.—Climate of the tertiary period.—On the causes of the absence
of recent conchiferous deposits on the coast of S. America.—On the
contemporaneous deposition and preservation of sedimentary formations.
RIO NEGRO.
I can add little to the details given by M. d’Orbigny on the sandstone
formation of this district. (“Voyage” Part Geolog. pages 57-65.) The
cliffs to the south of the river are about two hundred feet in height,
and are composed of sandstone of various tints and degrees of hardness.
One layer, which thinned out at both ends, consisted of earthy matter,
of a pale reddish colour, with some gypsum, and very like (I speak
after comparison of the specimens brought home) Pampean mud: above this
was a layer of compact marly rock with dendritic manganese. Many blocks
of a conglomerate of pumice-pebbles embedded in hard sandstone were
strewed at the foot of the cliff, and had evidently fallen from above.
A few miles N.E. of the town, I found, low down in the sandstone, a
bed, a few inches in thickness, of a white, friable, harsh-feeling
sediment, which adheres to the tongue, is of easy fusibility, and of
little specific gravity; examined under the microscope, it is seen to
be pumiceous tuff, formed of broken transparent crystals. In the cliffs
south of the river, there is, also, a thin layer of nearly similar
nature, but finer grained, and not so white; it might easily have been
mistaken for a calcareous tuff, but it contains no lime: this substance
precisely resembles a most widely extended and thick formation in
Southern Patagonia, hereafter to be described, and which is remarkable
for being partially formed of infusoria. These beds, conjointly with
the conglomerate of pumice, are interesting, as showing the nature of
the volcanic action in the Cordillera during this old tertiary period.
In a bed at the base of the southern cliffs, M. d’Orbigny found two
extinct fresh-water shells, namely, a Unio and Chilina. This bed rested
on one with bones of an extinct rodent, namely, the Megamys
Patagoniensis; and this again on another with extinct marine shells.
The species found by M. d’Orbigny in different parts of this formation
consist of:—
1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fe, and
whole coast of Patagonia). 2. Ostrea Ferrarisi, d’Orbigny, “Voyage,
Pal.” 3. Ostrea Alvarezii, d’Orbigny, “Voyage, Pal.” (also at St. Fe,
and S. Josef). 4. Pecten Patagoniensis, d’Orbigny, “Voyage, Pal.” 5.
Venus Munsterii, d’Orbigny, “Voyage, Pal.” (also at St. Fe). 6. Arca
Bonplandiana, d’Orbigny, “Voyage, Pal.” (also at St. Fe).
According to M. d’Orbigny, the sandstone extends westward along the
coast as far as Port S. Antonio, and up the R. Negro far into the
interior: northward I traced it to the southern side of the Rio
Colorado, where it forms a low denuded plain. This formation, though
contemporaneous with that of the rest of Patagonia, is quite different
in mineralogical composition, being connected with it only by the one
thin white layer: this difference may be reasonably attributed to the
sediment brought down in ancient times by the Rio Negro; by which
agency, also, we can understand the presence of the fresh-water shells,
and of the bones of land animals. Judging from the identity of four of
the above shells, this formation is contemporaneous (as remarked by M.
d’Orbigny) with that under the Pampean deposit in Entre Rios and in
Banda Oriental. The gravel capping the sandstone plain, with its
calcareous cement and nodules of gypsum, is probably, from the reasons
given in the First Chapter, contemporaneous with the uppermost beds of
the Pampean formation on the upper plain north of the Colorado.
SAN JOSEF.
My examination here was very short: the cliffs are about a hundred feet
high; the lower third consists of yellowish-brown, soft, slightly
calcareous, muddy sandstone, parts of which when struck emit a fetid
smell. In this bed the great Ostraea Patagonica, often marked with
dendritic manganese and small coral-lines, were extraordinarily
numerous. I found here the following shells:—
1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fe and
whole coast of Patagonia). 2. Ostrea Alvarezii, d’Orbigny, “Voyage,
Pal.” (also at St. Fe and R. Negro). 3. Pecten Paranensis, d’Orbigny,
“Voyage, Pal.” (also at St. Fe, S. Julian, and Port Desire). 4. Pecten
Darwinianus, d’Orbigny, “Voyage, Pal.” (also at St. Fe). 5. Pecten
actinodes, G.B. Sowerby. 6. Terebratula Patagonica, G.B. Sowerby (also
S. Julian). 7. Casts of a Turritella.
The four first of these species occur at St. Fe in Entre Rios, and the
two first in the sandstone of the Rio Negro. Above this fossiliferous
mass, there is a stratum of very fine-grained, pale brown mudstone,
including numerous laminae of selenite. All the strata appear
horizontal, but when followed by the eye for a long distance, they are
seen to have a small easterly dip. On the surface we have the
porphyritic gravel, and on it sand with recent shells.
NUEVO GULF.
From specimens and notes given me by Lieutenant Stokes, it appears that
the lower bed consists of soft muddy sandstone, like that of S. Josef,
with many imperfect shells, including the Pecten Paranensis, d’Orbigny,
casts of a Turritella and Scutella. On this there are two strata of the
pale brown mudstone, also like that of S. Josef, separated by a
darker-coloured, more argillaceous variety, including the Ostrea
Patagonica. Professor Ehrenberg has examined this mudstone for me: he
finds in it three already known microscopic organisms, enveloped in a
fine-grained pumiceous tuff, which I shall have immediately to describe
in detail. Specimens brought to me from the uppermost bed, north of the
Rio Chupat, consist of this same substance, but of a whiter colour.
Tertiary strata, such as here described, appear to extend along the
whole coast between Rio Chupat and Port Desire, except where
interrupted by the underlying claystone porphyry, and by some
metamorphic rocks; these hard rocks, I may add, are found at intervals
over a space of about five degrees of latitude, from Point Union to a
point between Port S. Julian and S. Cruz, and will be described in the
ensuing chapter. Many gigantic specimens of the Ostraea Patagonica were
collected in the Gulf of St. George.
PORT DESIRE.
A good section of the lowest fossiliferous mass, about forty feet in
thickness, resting on claystone porphyry, is exhibited a few miles
south of the harbour. The shells sufficiently perfect to be recognised
consist of:—
1. Ostrea Patagonica, d’Orbigny, (also at St. Fe, and whole coast of
Patagonia). 2. Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (also at
St. Fe, S. Josef, S. Julian). 3. Pecten centralis, G.B. Sowerby (also
at S. Julian and S. Cruz). 4. Cucullaea alta, G.B. Sowerby (also at S.
Cruz). 5. Nucula ornata, G.B. Sowerby. 6. Turritella Patagonica, G.B.
Sowerby.
The fossiliferous strata, when not denuded, are conformably covered by
a considerable thickness of the fine-grained pumiceous mudstone,
divided into two masses: the lower half is very fine-grained, slightly
unctuous, and so compact as to break with a semi-conchoidal fracture,
though yielding to the nail; it includes laminae of selenite: the upper
half precisely resembles the one layer at the Rio Negro, and with the
exception of being whiter, the upper beds at San Josef and Nuevo Gulf.
In neither mass is there any trace to the naked eye of organic forms.
Taking the entire deposit, it is generally quite white, or yellowish,
or feebly tinted with green; it is either almost friable under the
finger, or as hard as chalk; it is of easy fusibility, of little
specific gravity, is not harsh to the touch, adheres to the tongue, and
when breathed on exhales a strong aluminous odour; it sometimes
contains a very little calcareous matter, and traces (besides the
included laminae) of gypsum. Under the microscope, according to
Professor Ehrenberg, it consists of minute, triturated, cellular,
glassy fragments of pumice, with some broken crystals.
(“Monatsberichten de konig. Akad. zu Berlin” vom April 1845.) In the
minute glassy fragments, Professor Ehrenberg recognises organic
structures, which have been affected by volcanic heat: in the specimens
from this place, and from Port S. Julian, he finds sixteen Polygastrica
and twelve Phytolitharia. Of these organisms, seven are new forms, the
others being previously known: all are of marine, and chiefly of
oceanic, origin. This deposit to the naked eye resembles the crust
which often appears on weathered surfaces of feldspathic rocks; it
likewise resembles those beds of earthy feldspathic matter, sometimes
interstratified with porphyritic rocks, as is the case in this very
district with the underlying purple claystone porphyry. From examining
specimens under a common microscope, and comparing them with other
specimens undoubtedly of volcanic origin, I had come to the same
conclusion with Professor Ehrenberg, namely, that this great deposit,
in its first origin, is of volcanic nature.
PORT S. JULIAN.
(FIGURE 17. SECTION OF THE STRATA EXHIBITED IN THE CLIFFS OF THE NINETY
FEET PLAIN AT PORT S. JULIAN.
(Section through beds from top to bottom: A, B, C, D, E, F.))
On the south side of the harbour, Figure 17 gives the nature of the
beds seen in the cliffs of the ninety feet plain. Beginning at the
top:—
1st, the earthy mass (AA), including the remains of the Macrauchenia,
with recent shells on the surface.
Second, the porphyritic shingle (B), which in its lower part is
interstratified (owing, I believe, to redisposition during denudation)
with the white pumiceous mudstone.
Third, this white mudstone, about twenty feet in thickness, and divided
into two varieties (C and D), both closely resembling the lower, fine-
grained, more unctuous and compact kind at Port Desire; and, as at that
place, including much selenite.
Fourth, a fossiliferous mass, divided into three main beds, of which
the uppermost is thin, and consists of ferruginous sandstone, with many
shells of the great oyster and Pecten Paranensis; the middle bed (E) is
a yellowish earthy sandstone abounding with Scutellae; and the lowest
bed (F) is an indurated, greenish, sandy clay, including large
concretions of calcareous sandstone, many shells of the great oyster,
and in parts almost made up of fragments of Balanidae. Out of these
three beds, I procured the following twelve species, of which the two
first were exceedingly numerous in individuals, as were the
Terebratulae and Turritellae in certain layers:—
1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fe, and
whole coast of Patagonia). 2. Pecten Paranensis, d’Orbigny, “Voyage,
Pal.” (St. Fe, S. Josef, Port Desire). 3. Pecten centralis, G.B.
Sowerby (also at Port Desire and S. Cruz). 4. Pecten geminatus, G.B.
Sowerby. 5. Terebratula Patagonica, G.B. Sowerby (also S. Josef). 6.
Struthiolaria ornata, G.B. Sowerby (also S. Cruz). 7. Fusus
Patagonicus, G.B. Sowerby. 8. Fusus Noachinus, G.B. Sowerby. 9.
Scalaria rugulosa, G.B. Sowerby. 10. Turritella ambulacrum, G.B.
Sowerby (also S. Cruz). 11. Pyrula, cast of, like P. ventricosa of
Sowerby, Tank Cat. 12. Balanus varians, G.B. Sowerby. 13. Scutella,
differing from the species from Nuevo Gulf.
At the head of the inner harbour of Port S. Julian, the fossiliferous
mass is not displayed, and the sea-cliffs from the water’s edge to a
height of between one and two hundred feet are formed of the white
pumiceous mudstone, which here includes innumerable, far-extended,
sometimes horizontal, sometimes inclined or vertical laminae of
transparent gypsum, often about an inch in thickness. Further inland,
with the exception of the superficial gravel, the whole thickness of
the truncated hills, which represent a formerly continuous plain 950
feet in height, appears to be formed of this white mudstone: here and
there, however, at various heights, thin earthy layers, containing the
great oyster, Pecten Paranensis and Turritella ambulacrum, are
interstratified; thus showing that the whole mass belongs to the same
epoch. I nowhere found even a fragment of a shell actually in the white
deposit, and only a single cast of a Turritella. Out of the eighteen
microscopic organisms discovered by Ehrenberg in the specimens from
this place, ten are common to the same deposit at Port Desire. I may
add that specimens of this white mudstone, with the same identical
characters were brought me from two points,—one twenty miles north of
S. Julian, where a wide gravel-capped plain, 350 feet in height, is
thus composed; and the other forty miles south of S. Julian, where, on
the old charts, the cliffs are marked as “Chalk Hills.”
SANTA CRUZ.
The gravel-capped cliffs at the mouth of the river are 355 feet in
height: the lower part, to a thickness of fifty or sixty feet, consists
of a more or less hardened, darkish, muddy, or argillaceous sandstone
(like the lowest bed of Port Desire), containing very many shells, some
silicified and some converted into yellow calcareous spar. The great
oyster is here numerous in layers; the Trigonocelia and Turritella are
also very numerous: it is remarkable that the Pecten Paranensis, so
common in all other parts of the coast, is here absent: the shells
consist of:—
1. Ostrea Patagonica, d’Orbigny; “Voyage Pal.” (also at St. Fe and
whole coast of Patagonia).
2. Pecten centralis, G.B. Sowerby (also P. Desire and S. Julian).
3. Venus meridionalis of G.B. Sowerby.
4. Crassatella Lyellii, G.B. Sowerby.
5. Cardium puelchum, G.B. Sowerby.
6. Cardita Patagonica, G.B. Sowerby.
7. Mactra rugata, G.B. Sowerby.
8. Mactra Darwinii, G.B. Sowerby.
9. Cucullaea alta, G.B. Sowerby (also P. Desire).
10. Trigonocelia insolita, G.B. Sowerby.
11. Nucula (?) glabra, G.B. Sowerby.
12. Crepidula gregaria, G.B. Sowerby.
13. Voluta alta, G.B. Sowerby.
14. Trochus collaris, G.B. Sowerby.
15. Natica solida (?), G.B. Sowerby
16. Struthiolaria ornata, G.B. Sowerby (also P. Desire).
17. Turritella ambulacrum, G.B. Sowerby (also P. S. Julian).
Imperfect fragments of the genera Byssoarca, Artemis, and Fusus.
The upper part of the cliff is generally divided into three great
strata, differing slightly in composition, but essentially resembling
the pumiceous mudstone of the places farther north; the deposit,
however, here is more arenaceous, of greater specific gravity, and not
so white: it is interlaced with numerous thin veins, partially or quite
filled with transverse fibres of gypsum; these fibres were too short to
reach across the vein, have their extremities curved or bent: in the
same veins with the gypsum, and likewise in separate veins as well as
in little nests, there is much powdery sulphate of magnesia (as
ascertained by Mr. Reeks) in an uncompressed form: I believe that this
salt has not heretofore been found in veins. Of the three beds, the
central one is the most compact, and more like ordinary sandstone: it
includes numerous flattened spherical concretions, often united like a
necklace, composed of hard calcareous sandstone, containing a few
shells: some of these concretions were four feet in diameter, and in a
horizontal line nine feet apart, showing that the calcareous matter
must have been drawn to the centres of attraction, from a distance of
four feet and a half on both sides. In the upper and lower
finer-grained strata, there were other concretions of a grey colour,
containing calcareous matter, and so fine-grained and compact, as
almost to resemble porcelain- rock: I have seen exactly similar
concretions in a volcanic tufaceous bed in Chiloe. Although in this
upper fine-grained strata, organic remains were very rare, yet I
noticed a few of the great oyster; and in one included soft ferruginous
layer, there were some specimens of the Cucullaea alta (found at Port
Desire in the lower fossiliferous mass) and of the Mactra rugata, which
latter shell has been partially converted into gypsum.
(FIGURE 18. SECTION OF THE PLAINS OF PATAGONIA, ON THE BANKS OF THE S.
CRUZ.
(Section through strata (from top to bottom)): Surface of plain with
erratic boulders; 1,146 feet above the sea. a. Gravel and boulders, 212
feet thick. b. Basaltic lava, 322 feet thick. c, d and e. Sedimentary
layers, bed of small pebbles and talus respectively, total 592 feet
thick. River of S. Cruz; here 280 feet above sea.)
In ascending the valley of the S. Cruz, the upper strata of the coast-
cliffs are prolonged, with nearly the same characters, for fifty miles:
at about this point, they begin in the most gradual and scarcely
perceptible manner, to be banded with white lines; and after ascending
ten miles farther, we meet with distinct thin layers of whitish,
greenish, and yellowish fine-grained, fusible sediments. At eighty
miles from the coast, in a cliff thus composed, there were a few layers
of ferruginous sandstone, and of an argillaceous sandstone with
concretions of marl like those in the Pampas. (At this spot, for a
space of three-quarters of a mile along the north side of the river,
and for a width of half a mile, there has been a great slip, which has
formed hills between sixty and seventy feet in height, and has tilted
the strata into highly inclined and even vertical positions. The strata
generally dipped at an angle of 45 degrees towards the cliff from which
they had slided. I have observed in slips, both on a small and large
scale, that this inward dip is very general. Is it due to the
hydrostatic pressure of water percolating with difficulty through the
strata acting with greater force at the base of the mass than against
the upper part?) At one hundred miles from the coast, that is at a
central point between the Atlantic and the Cordillera, we have the
section in Figure 18.
The upper half of the sedimentary mass, under the basaltic lava,
consists of innumerable zones of perfectly white bright green,
yellowish and brownish, fine-grained, sometimes incoherent, sedimentary
matter. The white, pumiceous, trachytic tuff-like varieties are of
rather greater specific gravity than the pumiceous mudstone on the
coast to the north; some of the layers, especially the browner ones,
are coarser, so that the broken crystals are distinguishable with a
weak lens. The layers vary in character in short distances. With the
exception of a few of the Ostrea Patagonica, which appeared to have
rolled down from the cliff above, no organic remains were found. The
chief difference between these layers taken as a whole, and the upper
beds both at the mouth of the river and on the coast northward, seems
to lie in the occasional presence of more colouring matter, and in the
supply having been intermittent; these characters, as we have seen,
very gradually disappear in descending the valley, and this fact may
perhaps be accounted for by the currents of a more open sea having
blended together the sediment from a distant and intermittent source.
The coloured layers in the foregoing section rest on a mass, apparently
of great thickness (but much hidden by the talus), of soft sandstone,
almost composed of minute pebbles, from one-tenth to two-tenths of an
inch in diameter, of the rocks (with the entire exception of the
basaltic lava) composing the great boulders on the surface of the
plain, and probably composing the neighbouring Cordillera. Five miles
higher up the valley, and again thirty miles higher up (that is twenty
miles from the nearest range of the Cordillera), the lower plain
included within the upper escarpments, is formed, as seen on the banks
of the river, of a nearly similar but finer-grained, more earthy,
laminated sandstone, alternating with argillaceous beds, and containing
numerous moderately sized pebbles of the same rocks, and some shells of
the great Ostrea Patagonica. (I found at both places, but not in situ,
quantities of coniferous and ordinary dicotyledonous silicified wood,
which was examined for me by Mr. R. Brown.) As most of these shells had
been rolled before being here embedded, their presence does not prove
that the sandstone belongs to the great Patagonian tertiary formation,
for they might have been redeposited in it, when the valley existed as
a sea-strait; but as amongst the pebbles there were none of basalt,
although the cliffs on both sides of the valley are composed of this
rock, I believe that the sandstone does belong to this formation. At
the highest point to which we ascended, twenty miles distant from the
nearest slope of the Cordillera, I could see the horizontally zoned
white beds, stretching under the black basaltic lava, close up to the
mountains; so that the valley of the S. Cruz gives a fair idea of the
constitution of the whole width of Patagonia.
BASALTIC LAVA OF THE S. CRUZ.
This formation is first met with sixty-seven miles from the mouth of
the river; thence it extends uninterruptedly, generally but not
exclusively on the northern side of the valley, close up to the
Cordillera. The basalt is generally black and fine-grained, but
sometimes grey and laminated; it contains some olivine, and high up the
valley much glassy feldspar, where, also, it is often amygdaloidal; it
is never highly vesicular, except on the sides of rents and on the
upper and lower, spherically laminated surfaces. It is often columnar;
and in one place I saw magnificent columns, each face twelve feet in
width, with their interstices filled up with calcareous tuff. The
streams rest conformably on the white sedimentary beds, but I nowhere
saw the actual junction; nor did I anywhere see the white beds actually
superimposed on the lava; but some way up the valley at the foot of the
uppermost escarpments, they must be thus superimposed. Moreover, at the
lowest point down the valley, where the streams thin out and terminate
in irregular projections, the spaces or intervals between these
projections are filled up to the level of the now denuded and
gravel-capped surfaces of the plains, with the white-zoned sedimentary
beds; proving that this matter continued to be deposited after the
streams had flowed. Hence we may conclude that the basalt is
contemporaneous with the upper parts of the great tertiary formation.
The lava where first met with is 130 feet in thickness: it there
consists of two, three, or perhaps more streams, divided from each
other by vesicular spheroids like those on the surface. From the
streams having, as it appears, extended to different distances, the
terminal points are of unequal heights. Generally the surface of the
basalt is smooth them in one part high up the valley, it was so uneven
and hummocky, that until I afterwards saw the streams extending
continuously on both sides of the valley up to a height of about three
thousand feet close to the Cordillera, I thought that the craters of
eruption were probably close at hand. This hummocky surface I believe
to have been caused by the crossing and heaping up of different
streams. In one place, there were several rounded ridges about twenty
feet in height, some of them as broad as high, and some broader, which
certainly had been formed whilst the lava was fluid, for in transverse
sections each ridge was seen to be concentrically laminated, and to be
composed of imperfect columns radiating from common centres, like the
spokes of wheels.
The basaltic mass where first met with is, as I have said, 130 feet in
thickness, and, thirty-five miles higher up the valley, it increases to
322 feet. In the first fourteen and a half miles of this distance, the
upper surface of the lava, judging from three measurements taken above
the level of the river (of which the apparently very uniform
inclination has been calculated from its total height at a point 135
miles from the mouth), slopes towards the Atlantic at an angle of only
0 degrees 7 minutes twenty seconds: this must be considered only as an
approximate measurement, but it cannot be far wrong. Taking the whole
thirty-five miles, the upper surface slopes at an angle of 0 degrees 10
minutes 53 seconds; but this result is of no value in showing the
inclination of any one stream, for halfway between the two points of
measurement, the surface suddenly rises between one hundred and two
hundred feet, apparently caused by some of the uppermost streams having
extended thus far and no farther. From the measurement made at these
two points, thirty-five miles apart, the mean inclination of the
sedimentary beds, over which the lava has flowed, is NOW (after
elevation from under the sea) only 0 degrees 7 minutes 52 seconds: for
the sake of comparison, it may be mentioned that the bottom of the
present sea in a line from the mouth of the S. Cruz to the Falkland
Islands, from a depth of seventeen fathoms to a depth of eighty-five
fathoms, declines at an angle of 0 degrees 1 minute 22 seconds; between
the beach and the depth of seventeen fathoms, the slope is greater.
From a point about half-way up the valley, the basaltic mass rises more
abruptly towards the foot of the Cordillera, namely, from a height of
1,204 feet, to about 3,000 feet above the sea.
This great deluge of lava is worthy, in its dimensions, of the great
continent to which it belongs. The aggregate streams have flowed from
the Cordillera to a distance (unparalleled, I believe, in any case yet
known) of about one hundred geographical miles. Near their furthest
extremity their total thickness is 130 feet, which increase thirty-five
miles farther inland, as we have just seen, to 322 feet. The least
inclination given by M. E. de Beaumont of the upper surface of a
lava-stream, namely 0 degrees 30 minutes, is that of the great
subaerial eruption in 1783 from Skaptar Jukul in Iceland; and M. E. de
Beaumont shows that it must have flowed down a mean inclination of less
than 0 degrees 20 minutes. (“Memoires pour servir” etc. pages 178 and
217.) But we now see that under the pressure of the sea, successive
streams have flowed over a smooth bottom with a mean inclination of not
more than 0 degrees 7 minutes 52 seconds; and that the upper surface of
the terminal portion (over a space of fourteen and a half miles) has an
inclination of not more than 0 degrees 7 minutes 20 seconds. If the
elevation of Patagonia has been greater nearer the Cordillera than near
the Atlantic (as is probable), then these angles are now all too large.
I must repeat, that although the foregoing measurements, which were all
carefully taken with the barometer, may not be absolutely correct, they
cannot be widely erroneous.
Southward of the S. Cruz, the cliffs of the 840 feet plain extend to
Coy Inlet, and owing to the naked patches of the white sediment, they
are said on the charts to be “like the coast of Kent.” At Coy Inlet the
high plain trends inland, leaving flat-topped outliers. At Port
Gallegos (latitude 51 degrees 35 minutes, and ninety miles south of S.
Cruz), I am informed by Captain Sulivan, R.N., that there is a
gravel-capped plain from two to three hundred feet in height, formed of
numerous strata, some fine-grained and pale-coloured, like the upper
beds at the mouth of the S. Cruz, others rather dark and coarser, so as
to resemble gritstones or tuffs; these latter include rather large
fragments of apparently decomposed volcanic rocks; there are, also,
included layers of gravel. This formation is highly remarkable, from
abounding with mammiferous remains, which have not as yet been examined
by Professor Owen, but which include some large, but mostly small,
species of Pachydermata, Edentata, and Rodentia. From the appearance of
the pale-coloured, fine-grained beds, I was inclined to believe that
they corresponded with the upper beds of the S. Cruz; but Professor
Ehrenberg, who has examined some of the specimens, informs me that the
included microscopical organisms are wholly different, being fresh and
brackish-water forms. Hence the two to three hundred feet plain at Port
Gallegos is of unknown age, but probably of subsequent origin to the
great Patagonian tertiary formation.
EASTERN TIERRA DEL FUEGO.
Judging from the height, the general appearance, and the white colour
of the patches visible on the hill sides, the uppermost plain, both on
the north and western side of the Strait of Magellan, and along the
eastern coast of Tierra del Fuego as far south as near Port St.
Polycarp, probably belongs to the great Patagonian tertiary formation,
These higher table- ranges are fringed by low, irregular, extensive
plains, belonging to the boulder formation (Described in the
“Geological Transactions” volume 6 page 415.), and composed of coarse
unstratified masses, sometimes associated (as north of C. Virgin’s)
with fine, laminated, muddy sandstones. The cliffs in Sebastian Bay are
200 feet in height, and are composed of fine sandstones, often in
curvilinear layers, including hard concretions of calcareous sandstone,
and layers of gravel. In these beds there are fragments of wood, legs
of crabs, barnacles encrusted with corallines still partially retaining
their colour, imperfect fragments of a Pholas distinct from any known
species, and of a Venus, approaching very closely to, but slightly
different in form from, the V. lenticularis, a species living on the
coast of Chile. Leaves of trees are numerous between the laminae of the
muddy sandstone; they belong, as I am informed by Dr. J.D. Hooker, to
three species of deciduous beech, different from the two species which
compose the great proportion of trees in this forest-clad land.
(“Botany of the Antarctic Voyage” page 212.) From these facts it is
difficult to conjecture, whether we here see the basal part of the
great Patagonian formation, or some later deposit.
A SUMMARY ON THE PATAGONIAN TERTIARY FORMATION.
Four out of the seven fossil shells, from St. Fe in Entre Rios, were
found by M. d’Orbigny in the sandstone of the Rio Negro, and by me at
San Josef. Three out of the six from San Josef are identical with those
from Port Desire and S. Julian, which two places have together fifteen
species, out of which three are common to both. Santa Cruz has
seventeen species, out of which five are common to Port Desire and S.
Julian. Considering the difference in latitude between these several
places, and the small number of species altogether collected, namely
thirty-six, I conceive the above proportional number of species in
common, is sufficient to show that the lower fossiliferous mass belongs
nearly, I do not say absolutely, to the same epoch. What this epoch may
be, compared with the European tertiary stages, M. d’Orbigny will not
pretend to determine. The thirty-six species (including those collected
by myself and by M. d’Orbigny) are all extinct, or at least unknown;
but it should be borne in mind, that the present coast consists of
shingle, and that no one, I believe, has dredged here for shells; hence
it is not improbable that some of the species may hereafter be found
living. Some few of the species are closely related with existing ones;
this is especially the case, according to M. d’Orbigny and Mr. Sowerby,
with the Fusus Patagonicus; and, according to Mr. Sowerby, with the
Pyrula, the Venus meridionalis, the Crepidula gregaria, and the
Turritella ambulacrum, and T. Patagonica. At least three of the genera,
namely, Cucullaea, Crassatella, and (as determined by Mr. Sowerby)
Struthiolaria, are not found in this quarter of the world; and
Trigonocelia is extinct. The evidence taken altogether indicates that
this great tertiary formation is of considerable antiquity; but when
treating of the Chilean beds, I shall have to refer again to this
subject.
The white pumiceous mudstone, with its abundant gypsum, belongs to the
same general epoch with the underlying fossiliferous mass, as may be
inferred from the shells included in the intercalated layers at Nuevo
Gulf, S. Julian, and S. Cruz. Out of the twenty-seven marine
microscopic structures found by Professor Ehrenberg in the specimens
from S. Julian and Port Desire, ten are common to these two places: the
three found at Nuevo Gulf are distinct. I have minutely described this
deposit, from its remarkable characters and its wide extension. From
Coy Inlet to Port Desire, a distance of 230 miles, it is certainly
continuous; and I have reason to believe that it likewise extends to
the Rio Chupat, Nuevo Gulf, and San Josef, a distance of 570 miles: we
have, also, seen that a single layer occurs at the Rio Negro. At Port
S. Julian it is from eight to nine hundred feet in thickness; and at S.
Cruz it extends, with a slightly altered character, up to the
Cordillera. From its microscopic structure, and from its analogy with
other formations in volcanic districts, it must be considered as
originally of volcanic origin: it may have been formed by the
long-continued attrition of vast quantities of pumice, or judging from
the manner in which the mass becomes, in ascending the valley of S.
Cruz, divided into variously coloured layers, from the long-continued
eruption of clouds of fine ashes. In either case, we must conclude,
that the southern volcanic orifices of the Cordillera, now in a dormant
state, were at about this period over a wide space, and for a great
length of time, in action. We have evidence of this fact, in the
latitude of the Rio Negro, in the sandstone-conglomerate with pumice,
and demonstrative proof of it, at S. Cruz, in the vast deluges of
basaltic lava: at this same tertiary period, also, there is distinct
evidence of volcanic action in Western Banda Oriental.
The Patagonian tertiary formation extends continuously, judging from
fossils alone, from S. Cruz to near the Rio Colorado, a distance of
above six hundred miles, and reappears over a wide area in Entre Rios
and Banda Oriental, making a total distance of 1,100 miles; but this
formation undoubtedly extends (though no fossils were collected) far
south of the S. Cruz, and, according to M. d’Orbigny, 120 miles north
of St. Fe. At S. Cruz we have seen that it extends across the
continent; being on the coast about eight hundred feet in thickness
(and rather more at S. Julian), and rising with the contemporaneous
lava-streams to a height of about three thousand feet at the base of
the Cordillera. It rests, wherever any underlying formation can be
seen, on plutonic and metamorphic rocks. Including the newer Pampean
deposit, and those strata in Eastern Tierra del Fuego of doubtful age,
as well as the boulder formation, we have a line of more than
twenty-seven degrees of latitude, equal to that from the Straits of
Gibraltar to the south of Iceland, continuously composed of tertiary
formations. Throughout this great space the land has been upraised,
without the strata having been in a single instance, as far as my means
of observation went, unequally tilted or dislocated by a fault.
TERTIARY FORMATIONS ON THE WEST COAST.
CHONOS ARCHIPELAGO.
The numerous islands of this group, with the exception of Lemus, Ypun,
consist of metamorphic schists; these two islands are formed of softish
grey and brown, fusible, often laminated sandstones, containing a few
pebbles, fragments of black lignite, and numerous mammillated
concretions of hard calcareous sandstone. Out of these concretions at
Ypun (latitude 40 degrees 30 minutes S.), I extracted the four
following extinct species of shells:—
1. Turritella suturalis, G.B. Sowerby (also Navidad). 2. Sigaretus
subglobosus, G.B. Sowerby (also Navidad). 3. Cytheraea (?) sulculosa
(?), G.B. Sowerby (also Chiloe and Huafo?). 4. Voluta, fragments of.
In the northern parts of this group there are some cliffs of gravel and
of the boulder formation. In the southern part (at P. Andres in Tres
Montes), there is a volcanic formation, probably of tertiary origin.
The lavas attain a thickness of from two to three hundred feet; they
are extremely variable in colour and nature, being compact, or
brecciated, or cellular, or amygdaloidal with zeolite, agate and bole,
or porphyritic with glassy albitic feldspar. There is also much
imperfect rubbly pitchstone, with the interstices charged with powdery
carbonate of lime apparently of contemporaneous origin. These lavas are
conformably associated with strata of breccia and of brown tuff
containing lignite. The whole mass has been broken up and tilted at an
angle of 45 degrees, by a series of great volcanic dikes, one of which
was thirty yards in breadth. This volcanic formation resembles one,
presently to be described, in Chiloe.
HUAFO.
This island lies between the Chonos and Chiloe groups: it is about
eight hundred feet high, and perhaps has a nucleus of metamorphic
rocks. The strata which I examined consisted of fine-grained muddy
sandstones, with fragments of lignite and concretions of calcareous
sandstone. I collected the following extinct shells, of which the
Turritella was in great numbers:—
1. Bulla cosmophila, G.B. Sowerby. 2. Pleurotoma subaequalis, G.B.
Sowerby. 3. Fusus cleryanus, d’Orbigny, “Voyage Pal.” (also at
Coquimbo). 4. Triton leucostomoides, G.B. Sowerby. 5. Turritella
Chilensis, G.B. Sowerby (also Mocha). 6. Venus, probably a distinct
species, but very imperfect. 7. Cytheraea (?) sulculosa (?), probably a
distinct species, but very imperfect. 8. Dentalium majus, G.B. Sowerby.
CHILOE.
This fine island is about one hundred miles in length. The entire
southern part, and the whole western coast, consists of mica-schist,
which likewise is seen in the ravines of the interior. The central
mountains rise to a height of 3,000 feet, and are said to be partly
formed of granite and greenstone: there are two small volcanic
districts. The eastern coast, and large parts of the northern extremity
of the island are composed of gravel, the boulder formation, and
underlying horizontal strata. The latter are well displayed for twenty
miles north and south of Castro; they vary in character from common
sandstone to fine-grained, laminated mudstones: all the specimens which
I examined are easily fusible, and some of the beds might be called
volcanic grit-stones. These latter strata are perhaps related to a mass
of columnar trachyte which occurs behind Castro. The sandstone
occasionally includes pebbles, and many fragments and layers of
lignite; of the latter, some are apparently formed of wood and others
of leaves: one layer on the N.W. side of Lemuy is nearly two feet in
thickness. There is also much silicified wood, both common
dicotyledonous and coniferous: a section of one specimen in the
direction of the medullary rays has, as I am informed by Mr. R. Brown,
the discs in a double row placed alternately, and not opposite as in
the true Araucaria. I found marine remains only in one spot, in some
concretions of hard calcareous sandstone: in several other districts I
have observed that organic remains were exclusively confined to such
concretions; are we to account for this fact, by the supposition that
the shells lived only at these points, or is it not more probable that
their remains were preserved only where concretions were formed? The
shells here are in a bad state, they consist of:—
1. Tellinides (?) oblonga, G.B. Sowerby (a solenella in M. d’Orbigny’s
opinion). 2. Natica striolata, G.B. Sowerby. 3. Natica (?) pumila, G.B.
Sowerby. 4. Cytheraea (?) sulculosa, G.B. Sowerby (also Ypun and
Huafo?).
At the northern extremity of the island, near S. Carlos, there is a
large volcanic formation, between five and seven hundred feet in
thickness. The commonest lava is blackish-grey or brown, either
vesicular, or amygdaloidal with calcareous spar and bole: most even of
the darkest varieties fuse into a pale-coloured glass. The next
commonest variety is a rubbly, rarely well characterised pitchstone
(fusing into a white glass) which passes in the most irregular manner
into stony grey lavas. This pitchstone, as well as some purple
claystone porphyry, certainly flowed in the form of streams. These
various lavas often pass, at a considerable depth from the surface, in
the most abrupt and singular manner into wacke. Great masses of the
solid rock are brecciated, and it was generally impossible to discover
whether the recementing process had been an igneous or aqueous action.
(In a cliff of the hardest fragmentary mass, I found several tortuous,
vertical veins, varying in thickness from a few tenths of an inch to
one inch and a half, of a substance which I have not seen described. It
is glossy, and of a brown colour; it is thinly laminated, with the
laminae transparent and elastic; it is a little harder than calcareous
spar; it is infusible under the blowpipe, sometimes decrepitates, gives
out water, curls up, blackens, and becomes magnetic. Borax easily
dissolves a considerable quantity of it, and gives a glass tinged with
green. I have no idea what its true nature is. On first seeing it, I
mistook it for lignite!) The beds are obscurely separated from each
other; they are sometimes parted by seams of tuff and layers of
pebbles. In one place they rested on, and in another place were capped
by, tuffs and girt-stones, apparently of submarine origin.
The neighbouring peninsula of Lacuy is almost wholly formed of
tufaceous deposits, connected probably in their origin with the
volcanic hills just described. The tuffs are pale-coloured, alternating
with laminated mudstones and sandstones (all easily fusible), and
passing sometimes into fine-grained white beds strikingly resembling
the great upper infusorial deposit of Patagonia, and sometimes into
brecciolas with pieces of pumice in the last stage of decay; these
again pass into ordinary coarse breccias and conglomerates of hard
rocks. Within very short distances, some of the finer tuffs often
passed into each other in a peculiar manner, namely, by irregular
polygonal concretions of one variety increasing so much and so suddenly
in size, that the second variety, instead of any longer forming the
entire mass, was left merely in thin veins between the concretions. In
a straight line of cliffs, at Point Tenuy, I examined the following
remarkable section (Figure 19):—
(FIGURE 19.)
On the left hand, the lower part (AA) consists of regular, alternating
strata of brown tuffs and greenish laminated mudstone, gently inclined
to the right, and conformably covered by a mass (B left) of a white,
tufaceous and brecciolated deposit. On the right hand, the whole cliff
(BB right) consists of the same white tufaceous matter, which on this
side presents scarcely a trace of stratification, but to the left
becomes very gradually and rather indistinctly divided into strata
quite conformable with the underlying beds (AA): moreover, a few
hundred yards further to the left, where the surface has been less
denuded, the tufaceous strata (B left) are conformably covered by
another set of strata, like the underlying ones (AA) of this section.
In the middle of the diagram, the beds (AA) are seen to be abruptly cut
off, and to abut against the tufaceous non-stratified mass; but the
line of junction has been accidentally not represented steep enough,
for I particularly noticed that before the beds had been tilted to the
right, this line must have been nearly vertical. It appears that a
current of water cut for itself a deep and steep submarine channel, and
at the same time or afterwards filled it up with the tufaceous and
brecciolated matter, and spread the same over the surrounding submarine
beds; the matter becoming stratified in these more distant and less
troubled parts, and being moreover subsequently covered up by other
strata (like AA) not shown in the diagram. It is singular that three of
the beds (of AA) are prolonged in their proper direction, as
represented, beyond the line of junction into the white tufaceous
matter: the prolonged portions of two of the beds are rounded; in the
third, the terminal fragment has been pushed upwards: how these beds
could have been left thus prolonged, I will not pretend to explain. In
another section on the opposite side of a promontory, there was at the
foot of this same line of junction, that is at the bottom of the old
submarine channel, a pile of fragments of the strata (AA), with their
interstices filled up with white tufaceous matter: this is exactly what
might have been anticipated under such circumstances.
(FIGURE 20. GROUND PLAN SHOWING THE RELATION BETWEEN VEINS AND
CONCRETIONARY ZONES IN A MASS OF TUFF.)
The various tufaceous and other beds at this northern end of Chiloe
probably belong to about the same age with those near Castro, and they
contain, as there, many fragments of black lignite and of silicified
and pyritous wood, often embedded close together. They also contain
many and singular concretions: some are of hard calcareous sandstone,
in which it would appear that broken volcanic crystals and scales of
mica have been better preserved (as in the case of the organic remains
near Castro) than in the surrounding mass. Other concretions in the
white brecciola are of a hard, ferruginous, yet fusible, nature; they
are as round as cannon-balls, and vary from two or three inches to two
feet in diameter; their insides generally consist either of fine,
scarcely coherent volcanic sand (The frequent tendency in iron to form
hollow concretions or shell containing incoherent matter is singular;
D’Aubuisson (“Traite de Geogn.” tome 1 page 318) remarks on this
circumstance.), or of an argillaceous tuff; in this latter case, the
external crust was quite thin and hard. Some of these spherical balls
were encircled in the line of their equators, by a necklace-like row of
smaller concretions. Again there were other concretions, irregularly
formed, and composed of a hard, compact, ash- coloured stone, with an
almost porcelainous fracture, adhesive to the tongue, and without any
calcareous matter. These beds are, also, interlaced by many veins,
containing gypsum, ferruginous matter, calcareous spar, and agate. It
was here seen with remarkable distinctness, how intimately
concretionary action and the production of fissures and veins are
related together. Figure 20 is an accurate representation of a
horizontal space of tuff, about four feet long by two and a half in
width: the double lines represent the fissures partially filled with
oxide of iron and agate: the curvilinear lines show the course of the
innumerable, concentric, concretionary zones of different shades of
colour and of coarseness in the particles of tuff. The symmetry and
complexity of the arrangement gave the surface an elegant appearance.
It may be seen how obviously the fissures determine (or have been
determined by) the shape, sometimes of the whole concretion, and
sometimes only of its central parts. The fissures also determine the
curvatures of the long undulating zones of concretionary action. From
the varying composition of the veins and concretions, the amount of
chemical action which the mass has undergone is surprisingly great; and
it would likewise appear from the difference in size in the particles
of the concretionary zones, that the mass, also, has been subjected to
internal mechanical movements.
In the peninsula of Lacuy, the strata over a width of four miles have
been upheaved by three distinct, and some other indistinct, lines of
elevation, ranging within a point of north and south. One line, about
two hundred feet in height, is regularly anticlinal, with the strata
dipping away on both sides, at an angle of 15 degrees, from a central
“valley of elevation,” about three hundred yards in width. A second
narrow steep ridge, only sixty feet high, is uniclinal, the strata
throughout dipping westward; those on both flanks being inclined at an
angle of from ten to fifteen degrees; whilst those on the ridge dip in
the same direction at an angle of between thirty and forty degrees.
This ridge, traced northwards, dies away; and the beds at its terminal
point, instead of dipping westward, are inclined 12 degrees to the
north. This case interested me, as being the first in which I found in
South America, formations perhaps of tertiary origin, broken by lines
of elevation.
VALDIVIA: ISLAND OF MOCHA.
The formations of Chiloe seem to extend with nearly the same character
to Valdivia, and for some leagues northward of it: the underlying rocks
are micaceous schists, and are covered up with sandstone and other
sedimentary beds, including, as I was assured, in many places layers of
lignite. I did not land on Mocha (latitude 38 degrees 20 minutes), but
Mr. Stokes brought me specimens of the grey, fine-grained, slightly
calcareous sandstone, precisely like that of Huafo, containing lignite
and numerous Turritellae. The island is flat topped, 1,240 feet in
height, and appears like an outlier of the sedimentary beds on the
mainland. The few shells collected consist of:—
1. Turritella Chilensis, G.B. Sowerby (also at Huafo). 2. Fusus, very
imperfect, somewhat resembling F. subreflexus of Navidad, but probably
different. 3. Venus, fragments of.
CONCEPCION.
Sailing northward from Valdivia, the coast-cliffs are seen, first to
assume near the R. Tolten, and thence for 150 miles northward, to be
continued with the same mineralogical characters, immediately to be
described at Concepcion. I heard in many places of beds of lignite,
some of it fine and glossy, and likewise of silicified wood; near the
Tolten the cliffs are low, but they soon rise in height; and the
horizontal strata are prolonged, with a nearly level surface, until
coming to a more lofty tract between points Rumena and Lavapie. Here
the beds have been broken up by at least eight or nine parallel lines
of elevation, ranging E. or E.N.E. and W. or W.S.W. These lines can be
followed with the eye many miles into the interior; they are all
uniclinal, the strata in each dipping to a point between S. and S.S.E.
with an inclination in the central lines of about forty degrees, and in
the outer ones of under twenty degrees. This band of symmetrically
troubled country is about eight miles in width.
The island of Quiriquina, in the Bay of Concepcion, is formed of
various soft and often ferruginous sandstones, with bands of pebbles,
and with the lower strata sometimes passing into a conglomerate resting
on the underlying metamorphic schists. These beds include subordinate
layers of greenish impure clay, soft micaceous and calcareous
sandstones, and reddish friable earthy matter with white specks like
decomposed crystals of feldspar; they include, also, hard concretions,
fragments of shells, lignite, and silicified wood. In the upper part
they pass into white, soft sediments and brecciolas, very like those
described at Chiloe; as indeed is the whole formation. At Lirguen and
other places on the eastern side of the bay, there are good sections of
the lower sandstones, which are generally ferruginous, but which vary
in character, and even pass into an argillaceous nature; they contain
hard concretions, fragments of lignite, silicified wood, and pebbles
(of the same rocks with the pebbles in the sandstones of Quiriquina),
and they alternate with numerous, often very thin layers of imperfect
coal, generally of little specific gravity. The main bed here is three
feet thick; and only the coal of this one bed has a glossy fracture.
Another irregular, curvilinear bed of brown, compact lignite, is
remarkable for being included in a mass of coarse gravel. These
imperfect coals, when placed in a heap, ignite spontaneously. The
cliffs on this side of the bay, as well as on the island of Quiriquina,
are capped with red friable earth, which, as stated in the Second
Chapter, is of recent formation. The stratification in this
neighbourhood is generally horizontal; but near Lirguen the beds dip
N.W. at an angle of 23 degrees; near Concepcion they are also inclined:
at the northern end of Quiriquina they have been tilted at an angle of
30 degrees, and at the southern end at angles varying from 15 degrees
to 40 degrees: these dislocations must have taken place under the sea.
A collection of shells, from the island of Quiriquina, has been
described by M. d’Orbigny: they are all extinct, and from their generic
character, M. d’Orbigny inferred that they were of tertiary origin:
they consist of:—
1. Scalaria Chilensis, d’Orbigny, “Voyage, Part Pal.” 2. Natica
Araucana, d’Orbigny, “Voyage, Part Pal.” 3. Natica australis,
d’Orbigny, “Voyage, Part Pal.” 4. Fusus difficilis, d’Orbigny, “Voyage,
Part Pal.” 5. Pyrula longirostra, d’Orbigny, “Voyage, Part Pal.” 6.
Pleurotoma Araucana, d’Orbigny, “Voyage, Part Pal.” 7. Cardium auca,
d’Orbigny, “Voyage, Part Pal.” 8. Cardium acuticostatum, d’Orbigny,
“Voyage, Part Pal.” 9. Venus auca, d’Orbigny, “Voyage, Part Pal.” 10.
Mactra cecileana, d’Orbigny, “Voyage, Part Pal.” 11. Mactra Araucana,
d’Orbigny, “Voyage, Part Pal.” 12. Arca Araucana, d’Orbigny, “Voyage,
Part Pal.” 13. Nucula Largillierti, d’Orbigny, “Voyage, Part Pal.” 14.
Trigonia Hanetiana, d’Orbigny, “Voyage, Part Pal.”
During a second visit of the “Beagle” to Concepcion, Mr. Kent collected
for me some silicified wood and shells out of the concretions in the
sandstone from Tome, situated a short distance north of Lirguen. They
consist of:—
1. Natica australis, d’Orbigny, “Voyage, Part Pal.” 2. Mactra Araucana,
d’Orbigny, “Voyage, Part Pal.” 3. Trigonia Hanetiana, d’Orbigny,
“Voyage, Part Pal.” 4. Pecten, fragments of, probably two species, but
too imperfect for description. 5. Baculites vagina, E. Forbes. 6.
Nautilus d’Orbignyanus, E. Forbes.
Besides these shells, Captain Belcher found here an Ammonite, nearly
three feet in diameter, and so heavy that he could not bring it away;
fragments are deposited at Haslar Hospital: he also found the
silicified vertebrae of some very large animal. (“Zoology of Captain
Beechey’s Voyage” page 163.) From the identity in mineralogical nature
of the rocks, and from Captain Belcher’s minute description of the
coast between Lirguen and Tome, the fossiliferous concretions at this
latter place certainly belong to the same formation with the beds
examined by myself at Lirguen; and these again are undoubtedly the same
with the strata of Quiriquina; moreover; the three first of the shells
from Tome, though associated in the same concretions with the Baculite,
are identical with the species from Quiriquina. Hence all the sandstone
and lignitiferous beds in this neighbourhood certainly belong to the
same formation. Although the generic character of the Quiriquina
fossils naturally led M. d’Orbigny to conceive that they were of
tertiary origin, yet as we now find them associated with the Baculites
vagina and with an Ammonite, we must, in the opinion of M. d’Orbigny,
and if we are guided by the analogy of the northern hemisphere, rank
them in the Cretaceous system. Moreover, the Baculites vagina, which is
in a tolerable state of preservation, appears to Professor E. Forbes
certainly to be identical with a species, so named by him, from
Pondicherry in India; where it is associated with numerous decidedly
cretaceous species, which approach most nearly to Lower Greensand or
Neocomian forms: this fact, considering the vast distance between Chile
and India, is truly surprising. Again, the Nautilus d’Orbignyanus, as
far as its imperfect state allows of comparison, resembles, as I am
informed by Professor Forbes, both in its general form and in that of
its chambers, two species from the Upper Greensand. It may be added
that every one of the above-named genera from Quiriquina, which have an
apparently tertiary character, are found in the Pondicherry strata.
There are, however, some difficulties on this view of the formations at
Concepcion being cretaceous, which I shall afterwards allude to; and I
will here only state that the Cardium auca is found also at Coquimbo,
the beds at which place, there can be no doubt, are tertiary.
NAVIDAD. (I was guided to this locality by the Report on M. Gay’s
“Geological Researches” in the “Annales des Scienc. Nat.” 1st series
tome 28.)
The Concepcion formation extends some distance northward, but how far I
know not; for the next point at which I landed was at Navidad, 160
miles north of Concepcion, and 60 miles south of Valparaiso. The cliffs
here are about eight hundred feet in height: they consist, wherever I
could examine them, of fine-grained, yellowish, earthy sandstones, with
ferruginous veins, and with concretions of hard calcareous sandstone.
In one part, there were many pebbles of the common metamorphic
porphyries of the Cordillera: and near the base of the cliff, I
observed a single rounded boulder of greenstone, nearly a yard in
diameter. I traced this sandstone formation beneath the superficial
covering of gravel, for some distance inland: the strata are slightly
inclined from the sea towards the Cordillera, which apparently has been
caused by their having been accumulated against or round outlying
masses of granite, of which some points project near the coast. The
sandstone contains fragments of wood, either in the state of lignite or
partially silicified, sharks’ teeth, and shells in great abundance,
both high up and low down the sea-cliffs. Pectunculus and Oliva were
most numerous in individuals, and next to them Turritella and Fusus. I
collected in a short time, though suffering from illness, the following
thirty-one species, all of which are extinct, and several of the genera
do not now range (as we shall hereafter show) nearly so far south:—
1. Gastridium cepa, G.B. Sowerby. 2. Monoceros, fragments of,
considered by M. d’Orbigny as a new species. 3. Voluta alta, G.B.
Sowerby (considered by M. d’Orbigny as distinct from the V. alta of
Santa Cruz). 4. Voluta triplicata, G.B. Sowerby. 5. Oliva dimidiata,
G.B. Sowerby. 6. Pleurotoma discors, G.B. Sowerby. 7. Pleurotoma
turbinelloides, G.B. Sowerby. 8. Fusus subreflexus, G.B. Sowerby. 9.
Fusus pyruliformis, G.B. Sowerby. 10. Fusus, allied to F. regularis
(considered by M. d’Orbigny as a distinct species). 11. Turritella
suturalis, G.B. Sowerby. 12. Turritella Patagonica, G.B. Sowerby
(fragments of). 13. Trochus laevis, G.B. Sowerby. 14. Trochus collaris,
G.B. Sowerby (considered by M. d’Orbigny as the young of the T.
laevis). 15. Cassis monilifer, G.B. Sowerby. 16. Pyrula distans, G.B.
Sowerby. 17. Triton verruculosus, G.B. Sowerby. 18. Sigaretus
subglobosus, G.B. Sowerby. 19. Natica solida, G.B. Sowerby. (It is
doubtful whether the Natica solida of S. Cruz is the same species with
this.) 20. Terebra undulifera, G.B. Sowerby. 21. Terebra costellata,
G.B. Sowerby. 22. Bulla (fragments of). 23. Dentalium giganteum, do.
24. Dentalium sulcosum, do. 25. Corbis (?) laevigata, do. 26. Cardium
multiradiatum, do. 27. Venus meridionalis, do. 28. Pectunculus dispar,
(?) Desh. (considered by M. d’Orbigny as a distinct species). 29, 30.
Cytheraea and Mactra, fragments of (considered by M. d’Orbigny as new
species). 31. Pecten, fragments of.
COQUIMBO.
(FIGURE 21. SECTION OF THE TERTIARY FORMATION AT COQUIMBO.
From Level of Sea to Surface of plain, 252 feet above sea, through
levels F, E, D and C:
F.—Lower sandstone, with concretions and silicified bones, with fossil
shells, all, or nearly all, extinct.
E.—Upper ferruginous sandstone, with numerous Balani, with fossil
shells, all, or nearly all, extinct.
C and D.—Calcareous beds with recent shells.
A.—Stratified sand in a ravine, also with recent shells.)
For more than two hundred miles northward of Navidad, the coast
consists of plutonic and metamorphic rocks, with the exception of some
quite insignificant superficial beds of recent origin. At Tonguay,
twenty-five miles south of Coquimbo, tertiary beds recommence. I have
already minutely described in the Second Chapter, the step-formed
plains of Coquimbo, and the upper calcareous beds (from twenty to
thirty feet in thickness) containing shells of recent species, but in
different proportions from those on the beach. There remains to be
described only the underlying ancient tertiary beds, represented in
Figure 21 by the letters F and E:—
I obtained good sections of bed F only in Herradura Bay: it consists of
soft whitish sandstone, with ferruginous veins, some pebbles of
granite, and concretionary layers of hard calcareous sandstone. These
concretions are remarkable from the great number of large silicified
bones, apparently of cetaceous animals, which they contain; and
likewise of a shark’s teeth, closely resembling those of the Carcharias
megalodon. Shells of the following species, of which the gigantic
Oyster and Perna are the most conspicuous, are numerously embedded in
the concretions:—
1. Bulla ambigua, d’Orbigny “Voyage” Pal. 2. Monoceros Blainvillii,
d’Orbigny “Voyage” Pal. 3. Cardium auca, d’Orbigny “Voyage” Pal. 4.
Panopaea Coquimbensis, d’Orbigny “Voyage” Pal. 5. Perna Gaudichaudi,
d’Orbigny “Voyage” Pal. 6. Artemis ponderosa; Mr. Sowerby can find no
distinguishing character between this fossil and the recent A.
ponderosa; it is certainly an Artemis, as shown by the pallial
impression. 7. Ostrea Patagonica (?); Mr. Sowerby can point out no
distinguishing character between this species and that so eminently
characteristic of the great Patagonian formation; but he will not
pretend to affirm that they are identical. 8. Fragments of a Venus and
Natica.
The cliffs on one side of Herradura Bay are capped by a mass of
stratified shingle, containing a little calcareous matter, and I did
not doubt that it belonged to the same recent formation with the gravel
on the surrounding plains, also cemented by calcareous matter, until to
my surprise, I found in the midst of it, a single thin layer almost
entirely composed of the above gigantic oyster.
At a little distance inland, I obtained several sections of the bed E,
which, though different in appearance from the lower bed F, belongs to
the same formation: it consists of a highly ferruginous sandy mass,
almost composed, like the lowest bed at Port S. Julian, of fragments of
Balanidae; it includes some pebbles, and layers of yellowish-brown
mudstone. The embedded shells consist of:—
1. Monoceros Blainvillii, d’Orbigny “Voyage” Pal. 2. Monoceros
ambiguus, G.B. Sowerby. 3. Anomia alternans, G.B. Sowerby. 4. Pecten
rudis, G.B. Sowerby. 5. Perna Gaudichaudi, d’Orbigny “Voyage” Pal. 6.
Ostrea Patagonica (?), d’Orbigny “Voyage” Pal. 7. Ostrea, small
species, in imperfect state; it appeared to me like a small kind now
living in, but very rare in the bay. 8. Mytilus Chiloensis; Mr. Sowerby
can find no distinguishing character between this fossil, as far as its
not very perfect condition allows of comparison, and the recent
species. 9. Balanus Coquimbensis, G.B. Sowerby. 10. Balanus psittacus?
King. This appears to Mr. Sowerby and myself identical with a very
large and common species now living on the coast.
The uppermost layers of this ferrugino-sandy mass are conformably
covered by, and impregnated to the depth of several inches with, the
calcareous matter of the bed D called losa: hence I at one time
imagined that there was a gradual passage between them; but as all the
species are recent in the bed D, whilst the most characteristic shells
of the uppermost layers of E are the extinct Perna, Pecten, and
Monoceros, I agree with M. d’Orbigny, that this view is erroneous, and
that there is only a mineralogical passage between them, and no gradual
transition in the nature of their organic remains. Besides the fourteen
species enumerated from these two lower beds, M. d’Orbigny has
described ten other species given to him from this locality; namely:—
1. Fusus Cleryanus, d’Orbigny “Voyage” Pal. 2. Fusus petitianus,
d’Orbigny “Voyage” Pal. 3. Venus hanetiana, d’Orbigny “Voyage” Pal. 4.
Venus incerta (?) d’Orbigny “Voyage” Pal. 5. Venus Cleryana, d’Orbigny
“Voyage” Pal. 6. Venus petitiana, d’Orbigny “Voyage” Pal. 7. Venus
Chilensis, d’Orbigny “Voyage” Pal. 8. Solecurtus hanetianus, d’Orbigny
“Voyage” Pal. 9. Mactra auca, d’Orbigny “Voyage” Pal. 10. Oliva serena,
d’Orbigny “Voyage” Pal.
Of these twenty-four shells, all are extinct, except, according to Mr.
Sowerby, the Artemis ponderosa, Mytilus Chiloensis, and probably the
great Balanus.
COQUIMBO TO COPIAPO.
A few miles north of Coquimbo, I met with the ferruginous, balaniferous
mass E with many silicified bones; I was informed that these silicified
bones occur also at Tonguay, south of Coquimbo: their number is
certainly remarkable, and they seem to take the place of the silicified
wood, so common on the coast-formations of Southern Chile. In the
valley of Chaneral, I again saw this same formation, capped with the
recent calcareous beds. I here left the coast, and did not see any more
of the tertiary formations, until descending to the sea at Copiapo:
here in one place I found variously coloured layers of sand and soft
sandstone, with seams of gypsum, and in another place, a comminuted
shelly mass, with layers of rotten-stone and seams of gypsum, including
many of the extinct gigantic oyster: beds with these oysters are said
to occur at English Harbour, a few miles north of Copiapo.
COAST OF PERU.
With the exception of deposits containing recent shells and of quite
insignificant dimensions, no tertiary formations have been observed on
this coast, for a space of twenty-two degrees of latitude north of
Copiapo, until coming to Payta, where there is said to be a
considerable calcareous deposit: a few fossils have been described by
M. d’Orbigny from this place, namely:—
1. Rostellaria Gaudichaudi, d’Orbigny “Voyage” Pal. 2. Pectunculus
Paytensis, d’Orbigny “Voyage” Pal. 3. Venus petitiana, d’Orbigny
“Voyage” Pal. 4. Ostrea Patagonica? This great oyster (of which
specimens have been given me) cannot be distinguished by Mr. Sowerby
from some of the varieties from Patagonia; though it would be hazardous
to assert it is the same with that species, or with that from Coquimbo.
CONCLUDING REMARKS.
The formations described in this chapter, have, in the case of Chiloe
and probably in that of Concepcion and Navidad, apparently been
accumulated in troughs formed by submarine ridges extending parallel to
the ancient shores of the continent; in the case of the islands of
Mocha and Huafo it is highly probable, and in that of Ypun and Lemus
almost certain, that they were accumulated round isolated rocky centres
or nuclei, in the same manner as mud and sand are now collecting round
the outlying islets and reefs in the West Indian Archipelago. Hence, I
may remark, it does not follow that the outlying tertiary masses of
Mocha and Huafo were ever continuously united at the same level with
the formations on the mainland, though they may have been of
contemporaneous origin, and been subsequently upraised to the same
height. In the more northern parts of Chile, the tertiary strata seem
to have been separately accumulated in bays, now forming the mouths of
valleys.
The relation between these several deposits on the shores of the
Pacific, is not nearly so clear as in the case of the tertiary
formations on the Atlantic. Judging from the form and height of the
land (evidence which I feel sure is here much more trustworthy than it
can ever be in such broken continents as that of Europe), from the
identity of mineralogical composition, from the presence of fragments
of lignite and of silicified wood, and from the intercalated layers of
imperfect coal, I must believe that the coast-formations from Central
Chiloe to Concepcion, a distance of 400 miles, are of the same age:
from nearly similar reasons, I suspect that the beds of Mocha, Huafo,
and Ypun, belong also to the same period. The commonest shell in Mocha
and Huafo is the same species of Turritella; and I believe the same
Cytheraea is found on the islands of Huafo, Chiloe, and Ypun; but with
these trifling exceptions, the few organic remains found at these
places are distinct. The numerous shells from Navidad, with the
exception of two, namely, the Sigaretus and Turritella found at Ypun,
are likewise distinct from those found in any other part of this coast.
Coquimbo has Cardium auca in common with Concepcion, and Fusus
Cleryanus with Huafo; I may add, that Coquimbo has Venus petitiana, and
a gigantic oyster (said by M. d’Orbigny also to be found a little south
of Concepcion) in common with Payta, though this latter place is
situated twenty-two degrees northward of latitude 27 degrees, to which
point the Coquimbo formation extends.
From these facts, and from the generic resemblance of the fossils from
the different localities, I cannot avoid the suspicion that they all
belong to nearly the same epoch, which epoch, as we shall immediately
see, must be a very ancient tertiary one. But as the Baculite,
especially considering its apparent identity with the Cretaceous
Pondicherry species, and the presence of an Ammonite, and the
resemblance of the Nautilus to two upper greensand species, together
afford very strong evidence that the formation of Concepcion is a
Secondary one; I will, in my remarks on the fossils from the other
localities, put on one side those from Concepcion and from Eastern
Chiloe, which, whatever their age may be, appear to me to belong to one
group. I must, however, again call attention to the fact that the
Cardium auca is found both at Concepcion and in the undoubtedly
tertiary strata of Coquimbo: nor should the possibility be overlooked,
that as Trigonia, though known in the northern hemisphere only as a
Secondary genus, has living representatives in the Australian seas, so
a Baculite, Ammonite, and Trigonia may have survived in this remote
part of the southern ocean to a somewhat later period than to the north
of the equator.
Before passing in review the fossils from the other localities, there
are two points, with respect to the formations between Concepcion and
Chiloe, which deserve some notice. First, that though the strata are
generally horizontal, they have been upheaved in Chiloe in a set of
parallel anticlinal and uniclinal lines ranging north and south,—in the
district near P. Rumena by eight or nine far-extended, most
symmetrical, uniclinal lines ranging nearly east and west,—and in the
neighbourhood of Concepcion by less regular single lines, directed both
N.E. and S.W., and N.W. and S.E. This fact is of some interest, as
showing that within a period which cannot be considered as very ancient
in relation to the history of the continent, the strata between the
Cordillera and the Pacific have been broken up in the same variously
directed manner as have the old plutonic and metamorphic rocks in this
same district. The second point is, that the sandstone between
Concepcion and Southern Chiloe is everywhere lignitiferous, and
includes much silicified wood; whereas the formations in Northern Chile
do not include beds of lignite or coal, and in place of the fragments
of silicified wood there are silicified bones. Now, at the present day,
from Cape Horn to near Concepcion, the land is entirely concealed by
forests, which thin out at Concepcion, and in Central and Northern
Chile entirely disappear. This coincidence in the distribution of the
fossil wood and the living forests may be quite accidental; but I
incline to take a different view of it; for, as the difference in
climate, on which the presence of forests depends, is here obviously in
chief part due to the form of the land, and as the Cordillera
undoubtedly existed when the lignitiferous beds were accumulating, I
conceive it is not improbable that the climate, during the
lignitiferous period, varied on different parts of the coast in a
somewhat similar manner as it now does. Looking to an earlier epoch,
when the strata of the Cordillera were depositing, there were islands
which even in the latitude of Northern Chile, where now all is
irreclaimably desert, supported large coniferous forests.
TABLE 4.
Column 1. Genera, with living and tertiary species on the west coast of
South America. (M. d’Orbigny states that the genus Natica is not found
on the coast of Chile; but Mr. Cuming found it at Valparaiso. Scalaria
was found at Valparaiso; Arca, at Iquique, in latitude 20, by Mr.
Cuming; Arca, also, was found by Captain King, at Juan Fernandez, in
latitude 33 degrees 30′S.)
Column 2. Latitudes, in which found fossil on the coasts of Chile and
Peru. (In degrees and minutes.)
Column 3. Southernmost latitude, in which found living on the west
coast of South America. (In degrees and minutes.)
Bulla : 30 to 43 30 : 12 near Lima.
Cassis : 34 : 1 37.
Pyrula : 34 (and 36 30 at Concepcion) : 5 Payta.
Fusus : 30 and 43 30 : 23 Mexillones; reappears at the St. of Magellan.
Pleurotoma : 34 to 43 30 : 2 18 St. Elena.
Terebra : 34 : 5 Payta.
Sigaretus : 34 to 44 30 : 12 Lima.
Anomia : 30 : 7 48.
Perna : 30 : 1 23 Xixappa.
Cardium : 30 to 34 (and 36 30 at Concepcion) : 5 Payta.
Artemis : 30 : 5 Payta.
Voluta : 34 to 44 30 : Mr. Cuming does not know of any species living
on the west coast, between the equator and latitude 43 south; from this
latitude a species is found as far south as Tierra del Fuego.
Seventy-nine species of fossil shells, in a tolerably recognisable
condition, from the coast of Chile and Peru, are described in this
volume, and in the Palaeontological part of M. d’Orbigny’s “Voyage”: if
we put on one side the twenty species exclusively found at Concepcion
and Chiloe, fifty-nine species from Navidad and the other specified
localities remain. Of these fifty-nine species only an Artemis, a
Mytilus and Balanus, all from Coquimbo, are (in the opinion of Mr.
Sowerby, but not in that of M. d’Orbigny) identical with living shells;
and it would certainly require a better series of specimens to render
this conclusion certain. Only the Turritella Chilensis from Huafo and
Mocha, the T. Patagonica and Venus meridionalis from Navidad, come very
near to recent South American shells, namely, the two Turritellas to T.
cingulata, and the Venus to V. exalbida: some few other species come
rather less near; and some few resemble forms in the older European
tertiary deposits: none of the species resemble secondary forms. Hence
I conceive there can be no doubt that these formations are tertiary,—a
point necessary to consider, after the case of Concepcion. The
fifty-nine species belong to thirty-two genera; of these, Gastridium is
extinct, and three or four of the genera (viz. Panopaea, Rostellaria,
Corbis (?), and I believe Solecurtus) are not now found on the west
coast of South America. Fifteen of the genera have on this coast living
representatives in about the same latitudes with the fossil species;
but twelve genera now range very differently to what they formerly did.
The idea of Table 4, in which the difference between the extension in
latitude of the fossil and existing species is shown, is taken from M.
d’Orbigny’s work; but the range of the living shells is given on the
authority of Mr. Cuming, whose long-continued researches on the
conchology of South America are well-known.
When we consider that very few, if any, of the fifty-nine fossil shells
are identical with, or make any close approach to, living species; when
we consider that some of the genera do not now exist on the west coast
of South America, and that no less than twelve genera out of the
thirty-two formerly ranged very differently from the existing species
of the same genera, we must admit that these deposits are of
considerable antiquity, and that they probably verge on the
commencement of the tertiary era. May we not venture to believe, that
they are of nearly contemporaneous origin with the Eocene formations of
the northern hemisphere?
Comparing the fossil remains from the coast of Chile (leaving out, as
before, Concepcion and Chiloe) with those from Patagonia, we may
conclude, from their generic resemblance, and from the small number of
the species which from either coast approach closely to living forms,
that the formations of both belong to nearly the same epoch; and this
is the opinion of M. D’Orbigny. Had not a single fossil shell been
common to the two coasts, it could not have been argued that the
formations belonged to different ages; for Messrs. Cuming and Hinds
have found, on the comparison of nearly two thousand living species
from the opposite sides of South America, only one in common, namely,
the Purpura lapillus from both sides of the Isthmus of Panama: even the
shells collected by myself amongst the Chonos Islands and on the coast
of Patagonia, are dissimilar, and we must descend to the apex of the
continent, to Tierra del Fuego, to find these two great conchological
provinces united into one. Hence it is remarkable that four or five of
the fossil shells from Navidad, namely, Voluta alta, Turritella
Patagonica, Trochus collaris, Venus meridionalis, perhaps Natica
solida, and perhaps the large oyster from Coquimbo, are considered by
Mr. Sowerby as identical with species from Santa Cruz and P. Desire. M.
d’Orbigny, however, admits the perfect identity only of the Trochus.
ON THE TEMPERATURE OF THE TERTIARY PERIOD.
As the number of the fossil species and genera from the western and
eastern coasts is considerable, it will be interesting to consider the
probable nature of the climate under which they lived. We will first
take the case of Navidad, in latitude 34 degrees, where thirty-one
species were collected, and which, as we shall presently see, must have
inhabited shallow water, and therefore will necessarily well exhibit
the effects of temperature. Referring to Table 4 we find that the
existing species of the genera Cassis, Pyrula, Pleurotoma, Terebra, and
Sigaretus, which are generally (though by no means invariably)
characteristic of warmer latitudes, do not at the present day range
nearly so far south on this line of coast as the fossil species
formerly did. Including Coquimbo, we have Perna in the same
predicament. The first impression from this fact is, that the climate
must formerly have been warmer than it now is; but we must be very
cautious in admitting this, for Cardium, Bulla, and Fusus (and, if we
include Coquimbo, Anomia and Artemis) likewise formerly ranged farther
south than they now do; and as these genera are far from being
characteristic of hot climates, their former greater southern range may
well have been owing to causes quite distinct from climate: Voluta,
again, though generally so tropical a genus, is at present confined on
the west coast to colder or more southern latitudes than it was during
the tertiary period. The Trochus collaris, moreover, and, as we have
just seen according to Mr. Sowerby, two or three other species,
formerly ranged from Navidad as far south as Santa Cruz in latitude 50
degrees. If, instead of comparing the fossils of Navidad, as we have
hitherto done, with the shells now living on the west coast of South
America, we compare them with those found in other parts of the world,
under nearly similar latitudes; for instance, in the southern parts of
the Mediterranean or of Australia, there is no evidence that the sea
off Navidad was formerly hotter than what might have been expected from
its latitude, even if it was somewhat warmer than it now is when cooled
by the great southern polar current. Several of the most tropical
genera have no representative fossils at Navidad; and there are only
single species of Cassis, Pyrula, and Sigaretus, two of Pleurotoma and
two of Terebra, but none of these species are of conspicuous size. In
Patagonia, there is even still less evidence in the character of the
fossils, of the climate having been formerly warmer. (It may be worth
while to mention that the shells living at the present day on this
eastern side of South America, in latitude 40 degrees, have perhaps a
more tropical character than those in corresponding latitudes on the
shores of Europe: for at Bahia Blanca and S. Blas, there are two fine
species of Voluta and four of Oliva.) As from the various reasons
already assigned, there can be little doubt that the formations of
Patagonia and at least of Navidad and Coquimbo in Chile, are the
equivalents of an ancient stage in the tertiary formations of the
northern hemisphere, the conclusion that the climate of the southern
seas at this period was not hotter than what might have been expected
from the latitude of each place, appears to me highly important; for we
must believe, in accordance with the views of Mr. Lyell, that the
causes which gave to the older tertiary productions of the quite
temperate zones of Europe a tropical character, WERE OF A LOCAL
CHARACTER AND DID NOT AFFECT THE ENTIRE GLOBE. On the other hand, I
have endeavoured to show, in the “Geological Transactions,” that, at a
much later period, Europe and North and South America were nearly
contemporaneously subjected to ice- action, and consequently to a
colder, or at least more equable, climate than that now characteristic
of the same latitudes.
ON THE ABSENCE OF EXTENSIVE MODERN CONCHIFEROUS DEPOSITS IN SOUTH
AMERICA; AND ON THE CONTEMPORANEOUSNESS OF THE OLDER TERTIARY DEPOSITS
AT DISTANT POINTS BEING DUE TO CONTEMPORANEOUS MOVEMENTS OF SUBSIDENCE.
Knowing from the researches of Professor E. Forbes, that molluscous
animals chiefly abound within a depth of 100 fathoms and under, and
bearing in mind how many thousand miles of both coasts of South America
have been upraised within the recent period by a slow, long-continued,
intermittent movement,—seeing the diversity in nature of the shores and
the number of shells now living on them,—seeing also that the sea off
Patagonia and off many parts of Chile, was during the tertiary period
highly favourable to the accumulation of sediment,—the absence of
extensive deposits including recent shells over these vast spaces of
coast is highly remarkable. The conchiferous calcareous beds at
Coquimbo, and at a few isolated points northward, offer the most marked
exception to this statement; for these beds are from twenty to thirty
feet in thickness, and they stretch for some miles along shore,
attaining, however, only a very trifling breadth. At Valdivia there is
some sandstone with imperfect casts of shells, which POSSIBLY may
belong to the recent period: parts of the boulder formation and the
shingle-beds on the lower plains of Patagonia probably belong to this
same period, but neither are fossiliferous: it also so happens that the
great Pampean formation does not include, with the exception of the
Azara, any mollusca. There cannot be the smallest doubt that the
upraised shells along the shores of the Atlantic and Pacific, whether
lying on the bare surface, or embedded in mould or in sand-hillocks,
will in the course of ages be destroyed by alluvial action: this
probably will be the case even with the calcareous beds of Coquimbo, so
liable to dissolution by rain-water. If we take into consideration the
probability of oscillations of level and the consequent action of the
tidal-waves at different heights, their destruction will appear almost
certain. Looking to an epoch as far distant in futurity as we now are
from the past Miocene period, there seems to me scarcely a chance,
under existing conditions, of the numerous shells now living in those
zones of depths most fertile in life, and found exclusively on the
western and south-eastern coasts of South America, being preserved to
this imaginary distant epoch. A whole conchological series will in time
be swept away, with no memorials of their existence preserved in the
earth’s crust.
Can any light be thrown on this remarkable absence of recent
conchiferous deposits on these coasts, on which, at an ancient tertiary
epoch, strata abounding with organic remains were extensively
accumulated? I think there can, namely, by considering the conditions
necessary for the preservation of a formation to a distant age. Looking
to the enormous amount of denudation which on all sides of us has been
effected,—as evidenced by the lofty cliffs cutting off on so many
coasts horizontal and once far-extended strata of no great antiquity
(as in the case of Patagonia),—as evidenced by the level surface of the
ground on both sides of great faults and dislocations,—by inland lines
of escarpments, by outliers, and numberless other facts, and by that
argument of high generality advanced by Mr. Lyell, namely, that every
SEDIMENTARY formation, whatever its thickness may be, and over however
many hundred square miles it may extend, is the result and the measure
of an equal amount of wear and tear of pre-existing formations;
considering these facts, we must conclude that, as an ordinary rule, a
formation to resist such vast destroying powers, and to last to a
distant epoch, must be of wide extent, and either in itself, or
together with superincumbent strata, be of great thickness. In this
discussion, we are considering only formations containing the remains
of marine animals, which, as before mentioned, live, with some
exceptions within (most of them much within) depths of 100 fathoms.
How, then, can a thick and widely extended formation be accumulated,
which shall include such organic remains? First, let us take the case
of the bed of the sea long remaining at a stationary level: under these
circumstances it is evident that CONCHIFEROUS strata can accumulate
only to the same thickness with the depth at which the shells can live;
on gently inclined coasts alone can they accumulate to any considerable
width; and from the want of superincumbent pressure, it is probable
that the sedimentary matter will seldom be much consolidated: such
formations have no very good chance, when in the course of time they
are upraised, of long resisting the powers of denudation. The chance
will be less if the submarine surface, instead of having remained
stationary, shall have gone on slowly rising during the deposition of
the strata, for in this case their total thickness must be less, and
each part, before being consolidated or thickly covered up by
superincumbent matter, will have had successively to pass through the
ordeal of the beach; and on most coasts, the waves on the beach tend to
wear down and disperse every object exposed to their action. Now, both
on the south-eastern and western shores of South America, we have had
clear proofs that the land has been slowly rising, and in the long
lines of lofty cliffs, we have seen that the tendency of the sea is
almost everywhere to eat into the land. Considering these facts, it
ceases, I think, to be surprising, that extensive recent conchiferous
deposits are entirely absent on the southern and western shores of
America.
Let us take the one remaining case, of the bed of the sea slowly
subsiding during a length of time, whilst sediment has gone on being
deposited. It is evident that strata might thus accumulate to any
thickness, each stratum being deposited in shallow water, and
consequently abounding with those shells which cannot live at great
depths: the pressure, also, I may observe, of each fresh bed would aid
in consolidating all the lower ones. Even on a rather steep coast,
though such must ever be unfavourable to widely extended deposits, the
formations would always tend to increase in breadth from the water
encroaching on the land. Hence we may admit that periods of slow
subsidence will commonly be most favourable to the accumulation of
CONCHIFEROUS deposits, of sufficient thickness, extension, and
hardness, to resist the average powers of denudation.
We have seen that at an ancient tertiary epoch, fossiliferous deposits
were extensively deposited on the coasts of South America; and it is a
very interesting fact, that there is evidence that these ancient
tertiary beds were deposited during a period of subsidence. Thus, at
Navidad, the strata are about eight hundred feet in thickness, and the
fossil shells are abundant both at the level of the sea and some way up
the cliffs; having sent a list of these fossils to Professor E. Forbes,
he thinks they must have lived in water between one and ten fathoms in
depth: hence the bottom of the sea on which these shells once lived
must have subsided at least 700 feet to allow of the superincumbent
matter being deposited. I must here remark, that, as all these and the
following fossil shells are extinct species, Professor Forbes
necessarily judges of the depths at which they lived only from their
generic character, and from the analogical distribution of shells in
the northern hemisphere; but there is no just cause from this to doubt
the general results. At Huafo the strata are about the same thickness,
namely, 800 feet, and Professor Forbes thinks the fossils found there
cannot have lived at a greater depth than fifty fathoms, or 300 feet.
These two points, namely, Navidad and Huafo, are 570 miles apart, but
nearly halfway between them lies Mocha, an island 1,200 feet in height,
apparently formed of tertiary strata up to its level summit, and with
many shells, including the same Turritella with that found at Huafo,
embedded close to the level of the sea. In Patagonia, shells are
numerous at Santa Cruz, at the foot of the 350 feet plain, which has
certainly been formed by the denudation of the 840 feet plain, and
therefore was originally covered by strata that number of feet in
thickness, and these shells, according to Professor Forbes, probably
lived at a depth of between seven and fifteen fathoms: at Port S.
Julian, sixty miles to the north, shells are numerous at the foot of
the ninety feet plain (formed by the denudation of the 950 feet plain),
and likewise occasionally at the height of several hundred feet in the
upper strata; these shells must have lived in water somewhere between
five and fifty fathoms in depth. Although in other parts of Patagonia I
have no direct evidence of shoal-water shells having been buried under
a great thickness of superincumbent submarine strata, yet it should be
borne in mind that the lower fossiliferous strata with several of the
same species of Mollusca, the upper tufaceous beds, and the high
summit-plain, stretch for a considerable distance southward, and for
hundreds of miles northward; seeing this uniformity of structure, I
conceive it may be fairly concluded that the subsidence by which the
shells at Santa Cruz and S. Julian were carried down and covered up,
was not confined to these two points, but was co-extensive with a
considerable portion of the Patagonian tertiary formation. In a
succeeding chapter it will be seen, that we are led to a similar
conclusion with respect to the secondary fossiliferous strata of the
Cordillera, namely, that they also were deposited during a long-
continued and great period of subsidence. From the foregoing reasoning,
and from the facts just given, I think we must admit the probability of
the following proposition: namely, that when the bed of the sea is
either stationary or rising, circumstances are far less favourable,
than when the level is sinking, to the accumulation of CONCHIFEROUS
deposits of sufficient thickness and extension to resist, when
upheaved, the average vast amount of denudation. This result appears to
me, in several respects, very interesting: every one is at first
inclined to believe that at innumerable points, wherever there is a
supply of sediment, fossiliferous strata are now forming, which at some
future distant epoch will be upheaved and preserved; but on the views
above given, we must conclude that this is far from being the case; on
the contrary, we require (1st), a long-continued supply of sediment;
(2nd), an extensive shallow area; and (3rd), that this area shall
slowly subside to a great depth, so as to admit the accumulation of a
widely extended thick mass of superincumbent strata. In how few parts
of the world, probably, do these conditions at the present day concur!
We can thus, also, understand the general want of that close sequence
in fossiliferous formations which we might theoretically have
anticipated; for, without we suppose a subsiding movement to go on at
the same spot during an enormous period, from one geological era to
another, and during the whole of this period sediment to accumulate at
the proper rate, so that the depth should not become too great for the
continued existence of molluscous animals, it is scarcely possible that
there should be a perfect sequence at the same spot in the fossil
shells of the two geological formations. (Professor H.D. Rogers, in his
excellent address to the Association of American Geologists
(“Silliman’s Journal” volume 47 page 277) makes the following remark:
“I question if we are at all aware how COMPLETELY the whole history of
all departed time lies indelibly recorded with the amplest minuteness
of detail in the successive sediments of the globe, how effectually, in
other words, every period of time HAS WRITTEN ITS OWN HISTORY,
carefully preserving every created form and every trace of action.” I
think the correctness of such remarks is more than doubtful, even if we
except (as I suppose he would) all those numerous organic forms which
contain no hard parts.) So far from a very long-continued subsidence
being probable, many facts lead to the belief that the earth’s surface
oscillates up and down; and we have seen that during the elevatory
movements there is but a small chance of DURABLE fossiliferous deposits
accumulating.
Lastly, these same considerations appear to throw some light on the
fact that certain periods appear to have been favourable to the
deposition, or at least to the preservation, of contemporaneous
formations at very distant points. We have seen that in South America
an enormous area has been rising within the recent period; and in other
quarters of the globe immense spaces appear to have risen
contemporaneously. From my examination of the coral- reefs of the great
oceans, I have been led to conclude that the bed of the sea has gone on
slowly sinking within the present era, over truly vast areas: this,
indeed, is in itself probable, from the simple fact of the rising areas
having been so large. In South America we have distinct evidence that
at nearly the same tertiary period, the bed of the sea off parts of the
coast of Chile and off Patagonia was sinking, though these regions are
very remote from each other. If, then, it holds good, as a general
rule, that in the same quarter of the globe the earth’s crust tends to
sink and rise contemporaneously over vast spaces, we can at once see,
that we have at distant points, at the same period, those very
conditions which appear to be requisite for the accumulation of
fossiliferous masses of sufficient extension, thickness, and hardness,
to resist denudation, and consequently to last unto an epoch distant in
futurity. (Professor Forbes has some admirable remarks on this subject,
in his “Report on the Shells of the Aegean Sea.” In a letter to Mr.
Maclaren (“Edinburgh New Philosophical Journal” January 1843), I
partially entered into this discussion, and endeavoured to show that it
was highly improbable, that upraised atolls or barrier-reefs, though of
great thickness, should, owing to their small extension or breadth, be
preserved to a distant future period.)
CHAPTER VI.
PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.
Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of
foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in,
decomposition of.—La Plata, metamorphic and old volcanic rocks of.—S.
Ventana.—Claystone porphyry formation of Patagonia; singular
metamorphic rocks; pseudo-dikes.—Falkland Islands, Palaeozoic fossils
of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of;
cleavage and foliation; form of land.—Chonos Archipelago, mica-schists,
foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, dikes,
successive formation of.—Central and Northern Chile.—Concluding remarks
on cleavage and foliation.—Their close analogy and similar
origin.—Stratification of metamorphic schists.—Foliation of intrusive
rocks.—Relation of cleavage and foliation to the lines of tension
during metamorphosis.
The metamorphic and plutonic formations of the several districts
visited by the “Beagle” will be here chiefly treated of, but only such
cases as appear to me new, or of some special interest, will be
described in detail; at the end of the chapter I will sum up all the
facts on cleavage and foliation,— to which I particularly attended.
BAHIA, BRAZIL: latitude 13 degrees south.
The prevailing rock is gneiss, often passing, by the disappearance of
the quartz and mica, and by the feldspar losing its red colour, into a
brilliantly grey primitive greenstone. Not unfrequently quartz and
hornblende are arranged in layers in almost amorphous feldspar. There
is some fine-grained syenitic granite, orbicularly marked by
ferruginous lines, and weathering into vertical, cylindrical holes,
almost touching each other. In the gneiss, concretions of granular
feldspar and others of garnets with mica occur. The gneiss is traversed
by numerous dikes composed of black, finely crystallised, hornblendic
rock, containing a little glassy feldspar and sometimes mica, and
varying in thickness from mere threads to ten feet: these threads,
which are often curvilinear, could sometimes be traced running into the
larger dikes. One of these dikes was remarkable from having been in two
or three places laterally disjointed, with unbroken gneiss interposed
between the broken ends, and in one part with a portion of the gneiss
driven, apparently whilst in a softened state, into its side or wall.
In several neighbouring places, the gneiss included angular, well-
defined, sometimes bent, masses of hornblende rock, quite like, except
in being more perfectly crystallised, that forming the dikes, and, at
least in one instance, containing (as determined by Professor Miller)
augite as well as hornblende. In one or two cases these angular masses,
though now quite separate from each other by the solid gneiss, had,
from their exact correspondence in size and shape, evidently once been
united; hence I cannot doubt that most or all of the fragments have
been derived from the breaking up of the dikes, of which we see the
first stage in the above- mentioned laterally disjointed one. The
gneiss close to the fragments generally contained many large crystals
of hornblende, which are entirely absent or rare in other parts: its
folia or laminae were gently bent round the fragments, in the same
manner as they sometimes are round concretions. Hence the gneiss has
certainly been softened, its composition modified, and its folia
arranged, subsequently to the breaking up of the dikes, these latter
also having been at the same time bent and softened. (Professor
Hitchcock “Geology of Massachusetts” volume 2 page 673, gives a closely
similar case of a greenstone dike in syenite.)
I must here take the opportunity of premising, that by the term
CLEAVAGE I imply those planes of division which render a rock,
appearing to the eye quite or nearly homogeneous, fissile. By the term
FOLIATION, I refer to the layers or plates of different mineralogical
nature of which most metamorphic schists are composed; there are, also,
often included in such masses, alternating, homogeneous, fissile layers
or folia, and in this case the rock is both foliated and has a
cleavage. By STRATIFICATION, as applied to these formations, I mean
those alternate, parallel, large masses of different composition, which
are themselves frequently either foliated or fissile,—such as the
alternating so-called strata of mica-slate, gneiss, glossy clay-slate,
and marble.
The folia of the gneiss within a few miles round Bahia generally strike
irregularly, and are often curvilinear, dipping in all directions at
various angles: but where best defined, they extended most frequently
in a N.E. by N. (or East 50 degrees N.) and S.W. by S. line,
corresponding nearly with the coast-line northwards of the bay. I may
add that Mr. Gardner found in several parts of the province of Ceara,
which lies between four and five hundred miles north of Bahia, gneiss
with the folia extending E. 45 degrees N.; and in Guyana according to
Sir R. Schomburgk, the same rock strikes E. 57 degrees N. Again,
Humboldt describes the gneiss-granite over an immense area in Venezuela
and even in Colombia, as striking E. 50 degrees N., and dipping to the
N.W. at an angle of fifty degrees. (Gardner “Geological Section of the
British Association” 1840. For Sir R. Schomburgk’s observations see
“Geographical Journal” 1842 page 190. See also Humboldt’s discussion on
Loxodrism in the “Personal Narrative.”) Hence all the observations
hitherto made tend to show that the gneissic rocks over the whole of
this part of the continent have their folia extending generally within
almost a point of the compass of the same direction. (I landed at only
one place north of Bahia, namely, at Pernambuco. I found there only
soft, horizontally stratified matter, formed from disintegrated
granitic rocks, and some yellowish impure limestone, probably of a
tertiary epoch. I have described a most singular natural bar of hard
sandstone, which protects the harbour, in the 19th volume 1841 page 258
of the “London and Edinburgh Philosophical Magazine.”
ABROLHOS ISLETS, Latitude 18 degrees S. off the coast of Brazil.
Although not strictly in place, I do not know where I can more
conveniently describe this little group of small islands. The lowest
bed is a sandstone with ferruginous veins; it weathers into an
extraordinary honeycombed mass; above it there is a dark-coloured
argillaceous shale; above this a coarser sandstone—making a total
thickness of about sixty feet; and lastly, above these sedimentary
beds, there is a fine conformable mass of greenstone, in some parts
having a columnar structure. All the strata, as well as the surface of
the land, dip at an angle of about 12 degrees to N. by W. Some of the
islets are composed entirely of the sedimentary, others of the trappean
rocks, generally, however, with the sandstone, cropping out on the
southern shores.)
RIO DE JANEIRO.
This whole district is almost exclusively formed of gneiss, abounding
with garnets, and porphyritic with large crystals, even three and four
inches in length, of orthoclase feldspar: in these crystals mica and
garnets are often enclosed. At the western base of the Corcovado, there
is some ferruginous carious quartz-rock; and in the Tijeuka range, much
fine- grained granite. I observed boulders of greenstone in several
places; and on the islet of Villegagnon, and likewise on the coast some
miles northward, two large trappean dikes. The porphyritic gneiss, or
gneiss- granite as it has been called by Humboldt, is only so far
foliated that the constituent minerals are arranged with a certain
degree of regularity, and may be said to have a “GRAIN,” but they are
not separated into distinct folia or laminae. There are, however,
several other varieties of gneiss regularly foliated, and alternating
with each other in so-called strata. The stratification and foliation
of the ordinary gneisses, and the foliation or “grain” of the
gneiss-granite, are parallel to each other, and generally strike within
a point of N.E. and S.W. dipping at a high angle (between 50 and 60
degrees) generally to S.E.: so that here again we meet with the strike
so prevalent over the more northern parts of this continent. The
mountains of gneiss-granite are to a remarkable degree abruptly
conical, which seems caused by the rock tending to exfoliate in thick,
conically concentric layers: these peaks resemble in shape those of
phonolite and other injected rocks on volcanic islands; nor is the
grain or foliation (as we shall afterwards see) any difficulty on the
idea of the gneiss-granite having been an intrusive rather than a
metamorphic formation. The lines of mountains, but not always each
separate hill, range nearly in the same direction with the foliation
and so-called stratification, but rather more easterly.
(FIGURE 22. FRAGMENT OF GNEISS EMBEDDED IN ANOTHER VARIETY OF THE SAME
ROCK.)
On a bare gently inclined surface of the porphyritic gneiss in Botofogo
Bay, I observed the appearance represented in Figure 22. A fragment
seven yards long and two in width, with angular and distinctly defined
edges, composed of a peculiar variety of gneiss with dark layers of
mica and garnets, is surrounded on all sides by the ordinary gneiss-
granite; both having been dislocated by a granitic vein. The folia in
the fragment and in the surrounding rock strike in the same N.N.E. and
S.S.W. line; but in the fragment they are vertical, whereas in the
gneiss-granite they dip at a small angle, as shown by the arrows, to
S.S.E. This fragment, considering its great size, its solitary
position, and its foliated structure parallel to that of the
surrounding rock, is, as far as I know, a unique case: and I will not
attempt any explanation of its origin.
The numerous travellers in this country, have all been greatly
surprised at the depth to which the gneiss and other granitic rocks, as
well as the talcose slates of the interior, have been decomposed. (Spix
and Martius have collected in an Appendix to their “Travels,” the
largest body of facts on this subject. See also some remarks by M. Lund
in his communications to the Academy at Copenhagen; and others by M.
Gaudichaud in Freycinet “Voyage.”) Near Rio, every mineral except the
quartz has been completely softened, in some places to a depth little
less than one hundred feet. (Dr. Benza describes granitic rock, “Madras
Journal of Literature” etc. October 183? page 246), in the
Neelgherries, decomposed to a depth of forty feet.) The minerals retain
their positions in folia ranging in the usual direction; and fractured
quartz veins may be traced from the solid rock, running for some
distance into the softened, mottled, highly coloured, argillaceous
mass. It is said that these decomposed rocks abound with gems of
various kinds, often in a fractured state, owing, as some have
supposed, to the collapse of geodes, and that they contain gold and
diamonds. At Rio, it appeared to me that the gneiss had been softened
before the excavation (no doubt by the sea) of the existing, broad,
flat-bottomed valleys; for the depth of decomposition did not appear at
all conformable with the present undulations of the surface. The
porphyritic gneiss, where now exposed to the air, seems to withstand
decomposition remarkably well; and I could see no signs of any tendency
to the production of argillaceous masses like those here described. I
was also struck with the fact, that where a bare surface of this rock
sloped into one of the quiet bays, there were no marks of erosion at
the level of the water, and the parts both beneath and above it
preserved a uniform curve. At Bahia, the gneiss rocks are similarly
decomposed, with the upper parts insensibly losing their foliation, and
passing, without any distinct line of separation, into a bright red
argillaceous earth, including partially rounded fragments of quartz and
granite. From this circumstance, and from the rocks appearing to have
suffered decomposition before the excavation of the valleys, I suspect
that here, as at Rio, the decomposition took place under the sea. The
subject appeared to me a curious one, and would probably well repay
careful examination by an able mineralogist.
THE NORTHERN PROVINCES OF LA PLATA.
According to some observations communicated to me by Mr. Fox, the coast
from Rio de Janeiro to the mouth of the Plata seems everywhere to be
granitic, with a few trappean dikes. At Port Alegre, near the boundary
of Brazil, there are porphyries and diorites. (M. Isabelle “Voyage a
Buenos Ayres” page 479.) At the mouth of the Plata, I examined the
country for twenty-five miles west, and for about seventy miles north
of Maldonado: near this town, there is some common gneiss, and much, in
all parts of the country, of a coarse-grained mixture of quartz and
reddish feldspar, often, however, assuming a little dark-green
imperfect hornblende, and then immediately becoming foliated. The
abrupt hillocks thus composed, as well as the highly inclined folia of
the common varieties of gneiss, strike N.N.E. or a little more
easterly, and S.S.W. Clay-slate is occasionally met with, and near the
L. del Potrero, there is white marble, rendered fissile from the
presence of hornblende, mica, and asbestus; the cleavage of these rocks
and their stratification, that is the alternating masses thus composed,
strike N.N.E. and S.S.W. like the foliated gneisses, and have an almost
vertical dip. The Sierra Larga, a low range five miles west of
Maldonado, consists of quartzite, often ferruginous, having an
arenaceous feel, and divided into excessively thin, almost vertical
laminae or folia by microscopically minute scales, apparently of mica,
and striking in the usual N.N.E. and S.S.W. direction. The range itself
is formed of one principal line with some subordinate ones; and it
extends with remarkable uniformity far northward (it is said even to
the confines of Brazil), in the same line with the vertically ribboned
quartz rock of which it is composed. The S. de Las Animas is the
highest range in the country; I estimated it at 1,000 feet; it runs
north and south, and is formed of feldspathic porphyry; near its base
there is a N.N.W. and S.S.E. ridge of a conglomerate in a highly
porphyritic basis.
Northward of Maldonado, and south of Las Minas, there is an E. and W.
hilly band of country, some miles in width, formed of siliceous
clay-slate, with some quartz, rock, and limestone, having a tortuous
irregular cleavage, generally ranging east and west. E. and S.E. of Las
Minas there is a confused district of imperfect gneiss and laminated
quartz, with the hills ranging in various directions, but with each
separate hill generally running in the same line with the folia of the
rocks of which it is composed: this confusion appears to have been
caused by the intersection of the [E. and W.] and [N.N.E. and S.S.W.]
strikes. Northward of Las Minas, the more regular northerly ranges
predominate: from this place to near Polanco, we meet with the
coarse-grained mixture of quartz and feldspar, often with the imperfect
hornblende, and then becoming foliated in a N. and S. line—with
imperfect clay-slate, including laminae of red crystallised
feldspar—with white or black marble, sometimes containing asbestus and
crystals of gypsum—with quartz-rock—with syenite—and lastly, with much
granite. The marble and granite alternate repeatedly in apparently
vertical masses: some miles northward of the Polanco, a wide district
is said to be entirely composed of marble. It is remarkable, how rare
mica is in the whole range of country north and westward of Maldonado.
Throughout this district, the cleavage of the clay-slate and marble—the
foliation of the gneiss and the quartz—the stratification or
alternating masses of these several rocks—and the range of the hills,
all coincide in direction; and although the country is only hilly, the
planes of division are almost everywhere very highly inclined or
vertical.
Some ancient submarine volcanic rocks are worth mentioning, from their
rarity on this eastern side of the continent. In the valley of the
Tapas (fifty or sixty miles N. of Maldonado) there is a tract three or
four miles in length, composed of various trappean rocks with glassy
feldspar—of apparently metamorphosed grit-stones—of purplish
amygdaloids with large kernels of carbonate of lime (Near the Pan de
Azucar there is some greenish porphyry, in one place amygdaloidal with
agate.)—and much of a harshish rock with glassy feldspar intermediate
in character between claystone porphyry and trachyte. This latter rock
was in one spot remarkable from being full of drusy cavities, lined
with quartz crystals, and arranged in planes, dipping at an angle of 50
degrees to the east, and striking parallel to the foliation of an
adjoining hill composed of the common mixture of quartz, feldspar, and
imperfect hornblende: this fact perhaps indicates that these volcanic
rocks have been metamorphosed, and their constituent parts rearranged,
at the same time and according to the same laws, with the granitic and
metamorphic formations of this whole region. In the valley of the
Marmaraya, a few miles south of the Tapas, a band of trappean and
amygdaloidal rock is interposed between a hill of granite and an
extensive surrounding formation of red conglomerate, which (like that
at the foot of the S. Animas) has its basis porphyritic with crystals
of feldspar, and which hence has certainly suffered metamorphosis.
MONTE VIDEO.
The rocks here consist of several varieties of gneiss, with the
feldspar often yellowish, granular and imperfectly crystallised,
alternating with, and passing insensibly into, beds, from a few yards
to nearly a mile in thickness, of fine or coarse grained, dark-green
hornblendic slate; this again often passing into chloritic schist.
These passages seem chiefly due to changes in the mica, and its
replacement by other minerals. At Rat Island I examined a mass of
chloritic schist, only a few yards square, irregularly surrounded on
all sides by the gneiss, and intricately penetrated by many curvilinear
veins of quartz, which gradually BLEND into the gneiss: the cleavage of
the chloritic schist and the foliation of the gneiss were exactly
parallel. Eastward of the city there is much fine- grained,
dark-coloured gneiss, almost assuming the character of hornblende-
slate, which alternates in thin laminae with laminae of quartz, the
whole mass being transversely intersected by numerous large veins of
quartz: I particularly observed that these veins were absolutely
continuous with the alternating laminae of quartz. In this case and at
Rat Island, the passage of the gneiss into imperfect hornblendic or
into chloritic slate, seemed to be connected with the segregation of
the veins of quartz. (Mr. Greenough page 78 “Critical Examination”
etc., observes that quartz in mica-slate sometimes appears in beds and
sometimes in veins. Von Buch also in his “Travels in Norway” page 236,
remarks on alternating laminae of quartz and hornblende-slate replacing
mica-schist.)
The Mount, a hill believed to be 450 feet in height, from which the
place takes its name, is much the highest land in this neighbourhood:
it consists of hornblendic slate, which (except on the eastern and
disturbed base) has an east and west nearly vertical cleavage; the
longer axis of the hill also ranges in this same line. Near the summit
the hornblende-slate gradually becomes more and more coarsely
crystallised, and less plainly laminated, until it passes into a heavy,
sonorous greenstone, with a slaty conchoidal fracture; the laminae on
the north and south sides near the summit dip inwards, as if this upper
part had expanded or bulged outwards. This greenstone must, I conceive,
be considered as metamorphosed hornblende- slate. The Cerrito, the next
highest, but much less elevated point, is almost similarly composed. In
the more western parts of the province, besides gneiss, there is
quartz-rock, syenite, and granite; and at Colla, I heard of marble.
Near M. Video, the space which I more accurately examined was about
fifteen miles in an east and west line, and here I found the foliation
of the gneiss and the cleavage of the slates generally well developed,
and extending parallel to the alternating strata composed of the
gneiss, hornblendic and chloritic schists. These planes of division all
range within one point of east and west, frequently east by south and
west by north; their dip is generally almost vertical, and scarcely
anywhere under 45 degrees: this fact, considering how slightly
undulatory the surface of the country is, deserves attention. Westward
of M. Video, towards the Uruguay, wherever the gneiss is exposed, the
highly inclined folia are seen striking in the same direction; I must
except one spot where the strike was N.W. by W. The little Sierra de S.
Juan, formed of gneiss and laminated quartz, must also be excepted, for
it ranges between [N. to N.E.] and [S. to S.W.] and seems to belong to
the same system with the hills in the Maldonado district. Finally, we
have seen that, for many miles northward of Maldonado and for
twenty-five miles westward of it, as far as the S. de las Animas, the
foliation, cleavage, so-called stratification and lines of hills, all
range N.N.E. and S.S.W., which is nearly coincident with the adjoining
coast of the Atlantic. Westward of the S. de las Animas, as far as even
the Uruguay, the foliation, cleavage, and stratification (but not lines
of hills, for there are no defined ones) all range about E. by S. and
W. by N., which is nearly coincident with the direction of the northern
shore of the Plata; in the confused country near Las Minas, where these
two great systems appear to intersect each other, the cleavage,
foliation, and stratification run in various directions, but generally
coincide with the line of each separate hill.
SOUTHERN LA PLATA.
The first ridge, south of the Plata, which projects through the Pampean
formation, is the Sierra Tapalguen and Vulcan, situated 200 miles
southward of the district just described. This ridge is only a few
hundred feet in height, and runs from C. Corrientes in a W.N.W. line
for at least 150 miles into the interior: at Tapalguen, it is composed
of unstratified granular quartz, remarkable from forming tabular masses
and small plains, surrounded by precipitous cliffs: other parts of the
range are said to consist of granite: and marble is found at the S.
Tinta. It appears from M. Parchappe’s observations, that at Tandil
there is a range of quartzose gneiss, very like the rocks of the S.
Larga near Maldonado, running in the same N.N.E. and S.S.W. direction;
so that the framework of the country here is very similar to that on
the northern shore of the Plata. (M. d’Orbigny’s “Voyage” Part. Geolog.
page 46. I have given a short account of the peculiar forms of the
quartz hills of Tapalguen, so unusual in a metamorphic formation, in my
“Journal of Researches” 2nd edition page 116.)
The Sierra Guitru-gueyu is situated sixty miles south of the S.
Tapalguen: it consists of numerous parallel, sometimes blended together
ridges, about twenty-three miles in width, and five hundred feet in
height above the plain, and extending in a N.W. and S.E. direction.
Skirting round the extreme S.E. termination, I ascended only a few
points, which were composed of a fine-grained gneiss, almost composed
of feldspar with a little mica, and passing in the upper parts of the
hills into a rather compact purplish clay-slate. The cleavage was
nearly vertical, striking in a N.W. by W. and S.E. by E. line, nearly,
though not quite, coincident with the direction of the parallel ridges.
The Sierra Ventana lies close south of that of Guitru-gueyu; it is
remarkable from attaining a height, very unusual on this side of the
continent, of 3,340 feet. It consists up to its summit, of quartz,
generally pure and white, but sometimes reddish, and divided into thick
laminae or strata: in one part there is a little glossy clay-slate with
a tortuous cleavage. The thick layers of quartz strike in a W. 30
degrees N. line, dipping southerly at an angle of 45 degrees and
upwards. The principal line of mountains, with some quite subordinate
parallel ridges, range about W. 45 degrees N.: but at their S.E.
termination, only W. 25 degrees N. This Sierra is said to extend
between twenty and thirty leagues into the interior.
PATAGONIA.
With the exception perhaps of the hill of S. Antonio (600 feet high) in
the Gulf of S. Matias, which has never been visited by a geologist,
crystalline rocks are not met with on the coast of Patagonia for a
space of 380 miles south of the S. Ventana. At this point (latitude 43
degrees 50 minutes), at Points Union and Tombo, plutonic rocks are said
to appear, and are found, at rather wide intervals, beneath the
Patagonian tertiary formation for a space of about three hundred miles
southward, to near Bird Island, in latitude 48 degrees 56 minutes.
Judging from specimens kindly collected for me by Mr. Stokes, the
prevailing rock at Ports St. Elena, Camerones, Malaspina, and as far
south as the Paps of Pineda, is a purplish-pink or brownish claystone
porphyry, sometimes laminated, sometimes slightly vesicular, with
crystals of opaque feldspar and with a few grains of quartz; hence
these porphyries resemble those immediately to be described at Port
Desire, and likewise a series which I have seen from P. Alegre on the
southern confines of Brazil. This porphyritic formation further
resembles in a singularly close manner the lowest stratified formation
of the Cordillera of Chile, which, as we shall hereafter see, has a
vast range, and attains a great thickness. At the bottom of the Gulf of
St. George, only tertiary deposits appear to be present. At Cape
Blanco, there is quartz rock, very like that of the Falkland Islands,
and some hard, blue siliceous clay-slate.
At Port Desire there is an extensive formation of the claystone
porphyry, stretching at least twenty-five miles into the interior: it
has been denuded and deeply worn into gullies before being covered up
by the tertiary deposits, through which it here and there projects in
hills; those north of the bay being 440 feet in height. The strata have
in several places been tilted at small angles, generally either to
N.N.W. or S.S.E. By gradual passages and alternations, the porphyries
change incessantly in nature. I will describe only some of the
principal mineralogical changes, which are highly instructive, and
which I carefully examined. The prevailing rock has a compact purplish
base, with crystals of earthy or opaque feldspar, and often with grains
of quartz. There are other varieties, with an almost truly trachytic
base, full of little angular vesicles and crystals of glassy feldspar;
and there are beds of black perfect pitchstone, as well as of a
concretionary imperfect variety. On a casual inspection, the whole
series would be thought to be of the same plutonic or volcanic nature
with the trachytic varieties and pitchstone; but this is far from being
the case, as much of the porphyry is certainly of metamorphic origin.
Besides the true porphyries, there are many beds of earthy, quite white
or yellowish, friable, easily fusible matter, resembling chalk, which
under the microscope is seen to consist of minute broken crystals, and
which, as remarked in a former chapter, singularly resembles the upper
tufaceous beds of the Patagonian tertiary formation. This earthy
substance often becomes coarser, and contains minute rounded fragments
of porphyries and rounded grains of quartz, and in one case so many of
the latter as to resemble a common sandstone. These beds are sometimes
marked with true lines of aqueous deposition, separating particles of
different degrees of coarseness; in other cases there are parallel
ferruginous lines not of true deposition, as shown by the arrangement
of the particles, though singularly resembling them. The more indurated
varieties often include many small and some larger angular cavities,
which appear due to the removal of earthy matter: some varieties
contain mica. All these earthy and generally white stones insensibly
pass into more indurated sonorous varieties, breaking with a conchoidal
fracture, yet of small specific gravity; many of these latter varieties
assume a pale purple tint, being singularly banded and veined with
different shades, and often become plainly porphyritic with crystals of
feldspar. The formation of these crystals could be most clearly traced
by minute angular and often partially hollow patches of earthy matter,
first assuming a FIBROUS STRUCTURE, then passing into opaque
imperfectly shaped crystals, and lastly, into perfect glassy crystals.
When these crystals have appeared, and when the basis has become
compact, the rock in many places could not be distinguished from a true
claystone porphyry without a trace of mechanical structure.
In some parts, these earthy or tufaceous beds pass into jaspery and
into beautifully mottled and banded porcelain rocks, which break into
splinters, translucent at their edges, hard enough to scratch glass,
and fusible into white transparent beads: grains of quartz included in
the porcelainous varieties can be seen melting into the surrounding
paste. In other parts, the earthy or tufaceous beds either insensibly
pass into, or alternate with, breccias composed of large and small
fragments of various purplish porphyries, with the matrix generally
porphyritic: these breccias, though their subaqueous origin is in many
places shown both by the arrangement of their smaller particles and by
an oblique or current lamination, also pass into porphyries, in which
every trace of mechanical origin and stratification has been
obliterated.
Some highly porphyritic though coarse-grained masses, evidently of
sedimentary origin, and divided into thin layers, differing from each
other chiefly in the number of embedded grains of quartz, interested me
much from the peculiar manner in which here and there some of the
layers terminated in abrupt points, quite unlike those produced by a
layer of sediment naturally thinning out, and apparently the result of
a subsequent process of metamorphic aggregation. In another common
variety of a finer texture, the aggregating process had gone further,
for the whole mass consisted of quite short, parallel, often slightly
curved layers or patches, of whitish or reddish finely
granulo-crystalline feldspathic matter, generally terminating at both
ends in blunt points; these layers or patches further tended to pass
into wedge or almond-shaped little masses, and these finally into true
crystals of feldspar, with their centres often slightly drusy. The
series was so perfect that I could not doubt that these large crystals,
which had their longer axes placed parallel to each other, had
primarily originated in the metamorphosis and aggregation of
alternating layers of tuff; and hence their parallel position must be
attributed (unexpected though the conclusion may be), not to laws of
chemical action, but to the original planes of deposition. I am tempted
briefly to describe three other singular allied varieties of rock; the
first without examination would have passed for a stratified
porphyritic breccia, but all the included angular fragments consisted
of a border of pinkish crystalline feldspathic matter, surrounding a
dark translucent siliceous centre, in which grains of quartz not quite
blended into the paste could be distinguished: this uniformity in the
nature of the fragments shows that they are not of mechanical, but of
concretionary origin, having resulted perhaps from the self-breaking up
and aggregation of layers of indurated tuff containing numerous grains
of quartz,—into which, indeed, the whole mass in one part passed. The
second variety is a reddish non-porphyritic claystone, quite full of
spherical cavities, about half an inch in diameter, each lined with a
collapsed crust formed of crystals of quartz. The third variety also
consists of a pale purple non-porphyritic claystone, almost wholly
formed of concretionary balls, obscurely arranged in layers, of a less
compact and paler coloured claystone; each ball being on one side
partly hollow and lined with crystals of quartz.
PSEUDO-DIKES.
Some miles up the harbour, in a line of cliffs formed of slightly
metamorphosed tufaceous and porphyritic claystone beds, I observed
three vertical dikes, so closely resembling in general appearance
ordinary volcanic dikes, that I did not doubt, until closely examining
their composition, that they had been injected from below. The first is
straight, with parallel sides, and about four feet wide; it consists of
whitish, indurated tufaceous matter, precisely like some of the beds
intersected by it. The second dike is more remarkable; it is slightly
tortuous, about eighteen inches thick, and can be traced for a
considerable distance along the beach; it is of a purplish-red or brown
colour, and is formed chiefly of ROUNDED grains of quartz, with broken
crystals of earthy feldspar, scales of black mica, and minute fragments
of claystone porphyry, all firmly united together in a hard sparing
base. The structure of this dike shows obviously that it is of
mechanical and sedimentary origin; yet it thinned out upwards, and did
not cut through the uppermost strata in the cliffs. This fact at first
appears to indicate that the matter could not have been washed in from
above (Upfilled fissures are known to occur both in volcanic and in
ordinary sedimentary formations. At the Galapagos Archipelago “Volcanic
Islands” etc., there are some striking examples of pseudo-dikes
composed of hard tuff.); but if we reflect on the suction which would
result from a deep-seated fissure being formed, we may admit that if
the fissure were in any part open to the surface, mud and water might
well be drawn into it along its whole course. The third dike consisted
of a hard, rough, white rock, almost composed of broken crystals of
glassy feldspar, with numerous scales of black mica, cemented in a
scanty base; there was little in the appearance of this rock, to
preclude the idea of its having been a true injected feldspathic dike.
The matter composing these three pseudo-dikes, especially the second
one, appears to have suffered, like the surrounding strata, a certain
degree of metamorphic action; and this has much aided the deceptive
appearance. At Bahia, in Brazil, we have seen that a true injected
hornblendic dike, not only has suffered metamorphosis, but has been
dislocated and even diffused in the surrounding gneiss, under the form
of separate crystals and of fragments.
FALKLAND ISLANDS.
I have described these islands in a paper published in the third volume
of the “Geological Journal.” The mountain-ridges consist of quartz, and
the lower country of clay-slate and sandstone, the latter containing
Palaeozoic fossils. These fossils have been separately described by
Messrs. Morris and Sharpe: some of them resemble Silurian, and others
Devonian forms. In the eastern part of the group the several parallel
ridges of quartz extend in a west and east line; but further westward
the line becomes W.N.W. and E.S.E., and even still more northerly. The
cleavage-planes of the clay- slate are highly inclined, generally at an
angle of above 50 degrees, and often vertical; they strike almost
invariably in the same direction with the quartz ranges. The outline of
the indented shores of the two main islands, and the relative positions
of the smaller islets, accord with the strike both of the main axes of
elevation and of the cleavage of the clay- slate.
TIERRA DEL FUEGO.
My notes on the geology of this country are copious, but as they are
unimportant, and as fossils were found only in one district, a brief
sketch will be here sufficient. The east coast from the S. of Magellan
(where the boulder formation is largely developed) to St. Polycarp’s
Bay is formed of horizontal tertiary strata, bounded some way towards
the interior by a broad mountainous band of clay-slate. This great
clay-slate formation extends from St. Le Maire westward for 140 miles,
along both sides of the Beagle Channel to near its bifurcation. South
of this channel, it forms all Navarin Island, and the eastern half of
Hoste Island and of Hardy Peninsula; north of the Beagle Channel it
extends in a north-west line on both sides of Admiralty Sound to
Brunswick Peninsula in the St. of Magellan, and I have reason to
believe, stretches far up the eastern side of the Cordillera. The
western and broken side of Tierra del Fuego towards the Pacific is
formed of metamorphic schists, granite and various trappean rocks: the
line of separation between the crystalline and clay-slate formations
can generally be distinguished, as remarked by Captain King, by the
parallelism in the clay-slate districts of the shores and channels,
ranging in a line between [W. 20 degrees to 40 degrees N.] and [E. 20
degrees to 40 degrees S.]. (“Geographical Journal” volume 1 page 155.)
The clay-slate is generally fissile, sometimes siliceous or
ferruginous, with veins of quartz and calcareous spar; it often
assumes, especially on the loftier mountains, an altered feldspathic
character, passing into feldspathic porphyry: occasionally it is
associated with breccia and grauwacke. At Good Success Bay, there is a
little intercalated black crystalline limestone. At Port Famine much of
the clay-slate is calcareous, and passes either into a mudstone or into
grauwacke, including odd-shaped concretions of dark argillaceous
limestone. Here alone, on the shore a few miles north of Port Famine,
and on the summit of Mount Tarn (2,600 feet high), I found organic
remains; they consist of:—
1. Ancyloceras simplex, d’Orbigny “Pal Franc” Mount Tarn. 2. Fusus (in
imperfect state), d’Orbigny “Pal Franc” Mount Tarn. 3. Natica,
d’Orbigny “Pal Franc” Mount Tarn. 4. Pentacrimus, d’Orbigny “Pal Franc”
Mount Tarn. 5. Lucina excentrica, G.B. Sowerby, Port Famine. 6. Venus
(in imperfect state), G.B. Sowerby, Port Famine. 7. Turbinolia (?),
G.B. Sowerby, Port Famine. 8. Hamites elatior, G.B. Sowerby, Port
Famine.
M. d’Orbigny states that MM. Hombron and Grange found in this
neighbourhood an Ancyloceras, perhaps A. simplex, an Ammonite, a
Plicatula and Modiola. (“Voyage” Part Geolog. page 242.) M. d’Orbigny
believes from the general character of these fossils, and from the
Ancyloceras being identical (as far as its imperfect condition allows
of comparison) with the A. simplex of Europe, that the formation
belongs to an early stage of the Cretaceous system. Professor E.
Forbes, judging only from my specimens, concurs in the probability of
this conclusion. The Hamites elatior of the above list, of which a
description has been given by Mr. Sowerby, and which is remarkable from
its large size, has not been seen either by M. d’Orbigny or Professor
E. Forbes, as, since my return to England, the specimens have been
lost. The great clay-slate formation of Tierra del Fuego being
cretaceous, is certainly a very interesting fact,—whether we consider
the appearance of the country, which, without the evidence afforded by
the fossils, would form the analogy of most known districts, probably
have been considered as belonging to the Palaeozoic series,—or whether
we view it as showing that the age of this terminal portion of the
great axis of South America, is the same (as will hereafter be seen)
with the Cordillera of Chile and Peru.
The clay-slate in many parts of Tierra del Fuego, is broken by dikes
and by great masses of greenstone, often highly hornblendic (In a
greenstone-dike in the Magdalen Channel, the feldspar cleaved with the
angle of albite. This dike was crossed, as well as the surrounding
slate, by a large vein of quartz, a circumstance of unusual
occurrence.): almost all the small islets within the clay-slate
districts are thus composed. The slate near the dikes generally becomes
paler-coloured, harder, less fissile, of a feldspathic nature, and
passes into a porphyry or greenstone: in one case, however, it became
more fissile, of a red colour, and contained minute scales of mica,
which were absent in the unaltered rock. On the east side of Ponsonby
Sound some dikes composed of a pale sonorous feldspathic rock,
porphyritic with a little feldspar, were remarkable from their
number,—there being within the space of a mile at least one
hundred,—from their nearly equalling in bulk the intermediate
slate,—and more especially from the excessive fineness (like the finest
inlaid carpentry) and perfect parallelism of their junctions with the
almost vertical laminae of clay-slate. I was unable to persuade myself
that these great parallel masses had been injected, until I found one
dike which abruptly thinned out to half its thickness, and had one of
its walls jagged, with fragments of the slate embedded in it.
In Southern Tierra del Fuego, the clay-slate towards its S.W. boundary,
becomes much altered and feldspathic. Thus on Wollaston Island slate
and grauwacke can be distinctly traced passing into feldspathic rocks
and greenstones, including iron pyrites and epidote, but still
retaining traces of cleavage with the usual strike and dip. One such
metamorphosed mass was traversed by large vein-like masses of a
beautiful mixture (as ascertained by Professor Miller) of green
epidote, garnets, and white calcareous spar. On the northern point of
this same island, there were various ancient submarine volcanic rocks,
consisting of amygdaloids with dark bole and agate,—of basalt with
decomposed olivine—of compact lava with glassy feldspar,—and of a
coarse conglomerate of red scoriae, parts being amygdaloidal with
carbonate of lime. The southern part of Wollaston Island and the whole
of Hermite and Horn Islands, seem formed of cones of greenstone; the
outlying islets of Il Defenso and D. Raminez are said to consist of
porphyritic lava. (Determined by Professor Jameson. Weddell’s “Voyage”
page 169.) In crossing Hardy Peninsula, the slate still retaining
traces of its usual cleavage, passes into columnar feldspathic rocks,
which are succeeded by an irregular tract of trappean and basaltic
rocks, containing glassy feldspar and much iron pyrites: there is,
also, some harsh red claystone porphyry, and an almost true trachyte,
with needles of hornblende, and in one spot a curious slaty rock
divided into quadrangular columns, having a base almost like trachyte,
with drusy cavities lined by crystals, too imperfect, according to
Professor Miller, to be measured, but resembling Zeagonite. (See Mr.
Brooke’s Paper in the “London Philosophical Magazine” volume 10. This
mineral occurs in an ancient volcanic rock near Rome.) In the midst of
these singular rocks, no doubt of ancient submarine volcanic origin, a
high hill of feldspathic clay-slate projected, retaining its usual
cleavage. Near this point, there was a small hillock, having the aspect
of granite, but formed of white albite, brilliant crystals of
hornblende (both ascertained by the reflecting goniometer) and mica;
but with no quartz. No recent volcanic district has been observed in
any part of Tierra del Fuego.
Five miles west of the bifurcation of the Beagle Channel, the slate-
formation, instead of becoming, as in the more southern parts of Tierra
del Fuego, feldspathic, and associated with trappean or old volcanic
rocks, passes by alternations into a great underlying mass of fine
gneiss and glossy clay-slate, which at no great distance is succeeded
by a grand formation of mica-slate containing garnets. The folia of
these metamorphic schists strike parallel to the cleavage-planes of the
clay-slate, which have a very uniform direction over the whole of this
part of the country: the folia, however, are undulatory and tortuous,
whilst the cleavage- laminae of the slate are straight. These schists
compose the chief mountain-chain of Southern Tierra del Fuego, ranging
along the north side of the northern arm of the Beagle Channel, in a
short W.N.W. and E.S.E. line, with two points (Mounts Sarmiento and
Darwin) rising to heights of 6,800 and 6,900 feet. On the south-western
side of this northern arm of the Beagle Channel, the clay-slate is seen
with its STRATA dipping from the great chain, so that the metamorphic
schists here form a ridge bordered on each side by clay-slate. Further
north, however, to the west of this great range, there is no
clay-slate, but only gneiss, mica, and hornblendic slates, resting on
great barren hills of true granite, and forming a tract about sixty
miles in width. Again, westward of these rocks, the outermost islands
are of trappean formation, which, from information obtained during the
voyages of the “Adventure” and “Beagle,” seem, together with granite,
chiefly to prevail along the western coast as far north as the entrance
of the St. of Magellan (See the Paper by Captain King in the
“Geographical Journal”; also a Letter to Dr. Fitton in “Geological
Proceedings” volume 1 page 29; also some observations by Captain
Fitzroy “Voyages” volume 1 page 375. I am indebted also to Mr. Lyell
for a series of specimens collected by Lieutenant Graves.): a little
more inland, on the eastern side of Clarence Island and S. Desolation,
granite, greenstone, mica-slate, and gneiss appear to predominate. I am
tempted to believe, that where the clay-slate has been metamorphosed at
great depths beneath the surface, gneiss, mica- slate, and other allied
rocks have been formed, but where the action has taken place nearer the
surface, feldspathic porphyries, greenstones, etc., have resulted,
often accompanied by submarine volcanic eruptions.
Only one other rock, met with in both arms of the Beagle Channel,
deserves any notice, namely a granulo-crystalline mixture of white
albite, black hornblende (ascertained by measurement of the crystals,
and confirmed by Professor Miller), and more or less of brown mica, but
without any quartz. This rock occurs in large masses, closely
resembling in external form granite or syenite: in the southern arm of
the Channel, one such mass underlies the mica-slate, on which
clay-slate was superimposed: this peculiar plutonic rock which, as we
have seen, occurs also in Hardy Peninsula, is interesting, from its
perfect similarity with that (hereafter often to be referred to under
the name of andesite) forming the great injected axes of the Cordillera
of Chile.
The stratification of the clay-slate is generally very obscure, whereas
the cleavage is remarkably well defined: to begin with the extreme
eastern parts of Tierra del Fuego; the cleavage-planes near the St. of
Le Maire strike either W. and E. or W.S.W. and E.N.E., and are highly
inclined; the form of the land, including Staten Island, indicates that
the axes of elevation have run in this same line, though I was unable
to distinguish the planes of stratification. Proceeding westward, I
accurately examined the cleavage of the clay-slate on the northern,
eastern, and western sides (thirty-five miles apart) of Navarin Island,
and everywhere found the laminae ranging with extreme regularity,
W.N.W. and E.S.E., seldom varying more than one point of the compass
from this direction. (The clay-slate in this island was in many places
crossed by parallel smooth joints. Out of five cases, the angle of
intersection between the strike of these joints and that of the
cleavage-laminae was in two cases 45 degrees and in two others 79
degrees.) Both on the east and west coasts, I crossed at right angles
the cleavage-planes for a space of about eight miles, and found them
dipping at an angle of between 45 degrees and 90 degrees, generally to
S.S.W., sometimes to N.N.E., and often quite vertically. The S.S.W. dip
was occasionally succeeded abruptly by a N.N.E. dip, and this by a
vertical cleavage, or again by the S.S.W. dip; as in a lofty cliff on
the eastern end of the island the laminae of slate were seen to be
folded into very large steep curves, ranging in the usual W.N.W. line,
I suspect that the varying and opposite dips may possibly be accounted
for by the cleavage- laminae, though to the eye appearing straight,
being parts of large abrupt curves, with their summits cut off and worn
down.
In several places I was particularly struck with the fact, that the
fine laminae of the clay-slate, where cutting straight through the
bands of stratification, and therefore indisputably true
cleavage-planes, differed slightly in their greyish and greenish tints
of colour, in compactness, and in some of the laminae having a rather
more jaspery appearance than others. I have not seen this fact
recorded, and it appears to me important, for it shows that the same
cause which has produced the highly fissile structure, has altered in a
slight degree the mineralogical character of the rock in the same
planes. The bands of stratification, just alluded to, can be
distinguished in many places, especially in Navarin Island, but only on
the weathered surfaces of the slate; they consist of slightly
undulatory zones of different shades of colour and of thicknesses, and
resemble the marks (more closely than anything else to which I can
compare them) left on the inside of a vessel by the draining away of
some dirty slightly agitated liquid: no difference in composition,
corresponding with these zones, could be seen in freshly fractured
surfaces. In the more level parts of Navarin Island, these bands of
stratification were nearly horizontal; but on the flanks of the
mountains they were inclined from them, but in no instance that I saw
at a very high angle. There can, I think, be no doubt that these zones,
which appear only on the weathered surfaces, are the last vestiges of
the original planes of stratification, now almost obliterated by the
highly fissile and altered structure which the mass has assumed.
The clay-slate cleaves in the same W.N.W. and E.S.E. direction, as on
Navarin Island, on both sides of the Beagle Channel, on the eastern
side of Hoste Island, on the N.E. side of Hardy Peninsula, and on the
northern point of Wollaston Island; although in these two latter
localities the cleavage has been much obscured by the metamorphosed and
feldspathic condition of the slate. Within the area of these several
islands, including Navarin Island, the direction of the stratification
and of the mountain- chains is very obscure; though the mountains in
several places appeared to range in the same W.N.W. line with the
cleavage: the outline of the coast, however, does not correspond with
this line. Near the bifurcation of the Beagle Channel, where the
underlying metamorphic schists are first seen, they are foliated (with
some irregularities), in this same W.N.W. line, and parallel, as before
stated, to the main mountain-axis of this part of the country. Westward
of this main range, the metamorphic schists are foliated, though less
plainly, in the same direction, which is likewise common to the zone of
old erupted trappean rocks, forming the outermost islets. Hence the
area, over which the cleavage of the slate and the foliation of the
metamorphic schists extends with an average W.N.W. and E.S.E. strike,
is about forty miles in a north and south line, and ninety miles in an
east and west line.
Further northward, near Port Famine, the stratification of the
clay-slate and of the associated rocks, is well defined, and there
alone the cleavage and strata-planes are parallel. A little north of
this port there is an anticlinal axis ranging N.W. (or a little more
westerly) and S.E.: south of the port, as far as Admiralty Sound and
Gabriel Channel, the outline of the land clearly indicates the
existence of several lines of elevation in this same N.W. direction,
which, I may add, is so uniform in the western half of the St. of
Magellan, that, as Captain King has remarked, “a parallel ruler placed
on the map upon the projecting points of the south shore, and extended
across the strait, will also touch the headlands on the opposite
coast.” (“Geographical Journal” volume 1 page 170.) It would appear,
from Captain King’s observations, that over all this area the cleavage
extends in the same line. Deep-water channels, however, in all parts of
Tierra del Fuego have burst through the trammels both of stratification
and cleavage; most of them may have been formed during the elevation of
the land by long- continued erosion, but others, for instance the
Beagle Channel, which stretches like a narrow canal for 120 miles
obliquely through the mountains, can hardly have thus originated.
Finally, we have seen that in the extreme eastern point of Tierra del
Fuego, the cleavage and coast-lines extend W. and E. and even W.S.W.
and E.N.E.: over a large area westward, the cleavage, the main range of
mountains, and some subordinate ranges, but not the outlines of the
coast, strike W.N.W., and E.S.E.: in the central and western parts of
the St. of Magellan, the stratification, the mountain-ranges, the
outlines of the coast, and the cleavage all strike nearly N.W. and S.E.
North of the strait, the outline of the coast, and the mountains on the
mainland, run nearly north and south. Hence we see, at this southern
point of the continent, how gradually the Cordillera bend, from their
north and south course of so many thousand miles in length, into an E.
and even E.N.E. direction.
WEST COAST, FROM THE SOUTHERN CHONOS ISLANDS TO NORTHERN CHILE.
The first place at which we landed north of the St. of Magellan was
near Cape Tres Montes, in latitude 47 degrees S. Between this point and
the Northern Chonos Islands, a distance of 200 miles, the “Beagle”
visited several points, and specimens were collected for me from the
intermediate spaces by Lieutenant Stokes. The predominant rock is
mica-slate, with thick folia of quartz, very frequently alternating
with and passing into a chloritic, or into a black, glossy, often
striated, slightly anthracitic schist, which soils paper, and becomes
white under a great heat, and then fuses. Thin layers of feldspar,
swelling at intervals into well crystallised kernels, are sometimes
included in these black schists; and I observed one mass of the
ordinary black variety insensibly lose its fissile structure, and pass
into a singular mixture of chlorite, epidote, feldspar, and mica. Great
veins of quartz are numerous in the mica-schists; wherever these occur
the folia are much convoluted. In the southern part of the Peninsula of
Tres Montes, a compact altered feldspathic rock with crystals of
feldspar and grains of quartz is the commonest variety; this rock
exhibits occasionally traces of an original brecciated structure, and
often presents (like the altered state of Tierra del Fuego) traces of
cleavage- planes, which strike in the same direction with the folia of
mica-schist further northward. (The peculiar, abruptly conical form of
the hills in this neighbourhood, would have led any one at first to
have supposed that they had been formed of injected or intrusive rocks.
At Inchemo Island, a similar rock gradually becomes granulo-crystalline
and acquires scales of mica; and this variety at S. Estevan becomes
highly laminated, and though still exhibiting some rounded grains of
quartz, passes into the black, glossy, slightly anthracitic schist,
which, as we have seen, repeatedly alternates with and passes into the
micaceous and chloritic schists. Hence all the rocks on this line of
coast belong to one series, and insensibly vary from an altered
feldspathic clay-slate into largely foliated, true mica-schist.
The cleavage of the homogeneous schists, the foliation of those
composed of more or less distinct minerals in layers, and the planes of
alternation of the different varieties or so-called stratification, are
all parallel, and preserve over this 200 miles of coast a remarkable
degree of uniformity in direction. At the northern end of the group, at
Low’s Harbour, the well- defined folia of mica-schist everywhere ranged
within eight degrees (or less than one point of the compass) of N. 19
degrees W. and S. 19 degrees E.; and even the point of dip varied very
little, being always directed to the west and generally at an angle of
forty degrees; I should mention that I had here good opportunities of
observation, for I followed the naked rock on the beach, transversely
to the strike, for a distance of four miles and a half, and all the way
attended to the dip. Along the outer islands for 100 miles south of
Low’s Harbour, Lieutenant Stokes, during his boat- survey, kindly
observed for me the strike of the foliation, and he assures me that it
was invariably northerly, and the dip with one single exception to the
west. Further south at Vallenar Bay, the strike was almost universally
N. 25 degrees W. and the dip, generally at an angle of about 40 degrees
to W. 25 degrees S., but in some places almost vertical. Still farther
south, in the neighbourhood of the harbours of Anna Pink, S. Estevan
and S. Andres, and (judging from a distance) along the southern part of
Tres Montes, the foliation and cleavage extended in a line between [N.
11 degrees to 22 degrees W.] and [S. 11 degrees to 22 degrees E.]; and
the planes dipped generally westerly, but often easterly, at angles
varying from a gentle inclination to vertical. At A. Pink’s Harbour,
where the schists generally dipped easterly, wherever the angle became
very high, the strike changed from N. 11 degrees W. to even as much as
N. 45 degrees W.: in an analogous manner at Vallenar Bay, where the dip
was westerly (viz. on an average directed to W. 25 degrees S.), as soon
as the angle became very high, the planes struck in a line more than 25
degrees west of north. The average result from all the observations on
this 200 miles of coast, is a strike of N. 19 degrees W. and S. 19
degrees E.: considering that in each specified place my examination
extended over an area of several miles, and that Lieutenant Stokes’
observations apply to a length of 100 miles, I think this remarkable
uniformity is pretty well established. The prevalence, throughout the
northern half of this line of coast, of a dip in one direction, that is
to the west, instead of being sometimes west and sometimes east, is,
judging from what I have elsewhere seen, an unusual circumstance. In
Brazil, La Plata, the Falkland Islands, and Tierra del Fuego, there is
generally an obvious relation between the axis of elevation, the
outline of the coast, and the strike of the cleavage or foliation: in
the Chonos Archipelago, however, neither the minor details of the
coast-line, nor the chain of the Cordillera, nor the subordinate
transverse mountain-axes, accord with the strike of the foliation and
cleavage: the seaward face of the numerous islands composing this
Archipelago, and apparently the line of the Cordillera, range N. 11
degrees E., whereas, as we have just seen, the average strike of the
foliation is N. 19 degrees W.
There is one interesting exception to the uniformity in the strike of
the foliation. At the northern point of Tres Montes (latitude 45
degrees 52 minutes) a bold chain of granite, between two and three
thousand feet in height, runs from the coast far into the interior, in
an E.S.E. line, or more strictly E. 28 degrees S. and W. 28 degrees N.
(In the distance, other mountains could be seen apparently ranging
N.N.E. and S.S.W. at right angles to this one. I may add, that not far
from Vallenar Bay there is a fine range, apparently of granite, which
has burst through the mica-slate in a N.E. by E. and S.W. by S. line.)
In a bay, at the northern foot of this range, there are a few islets of
mica-slate, with the folia in some parts horizontal, but mostly
inclined at an average angle of 20 degrees to the north. On the
northern steep flank of the range, there are a few patches (some quite
isolated, and not larger than half a-crown!) of the mica-schist,
foliated with the same northerly dip. On the broad summit, as far as
the southern crest, there is much mica-slate, in some places even 400
feet in thickness, with the folia all dipping north, at angles varying
from 5 degrees to 20 degrees, but sometimes mounting up to 30 degrees.
The southern flank consists of bare granite. The mica-slate is
penetrated by small veins of granite, branching from the main body.
(The granite within these veins, as well as generally at the junction
with the mica-slate, is more quartzose than elsewhere. The granite, I
may add, is traversed by dikes running for a very great length in the
line of the mountains; they are composed of a somewhat laminated
eurite, containing crystals of feldspar, hornblende, and octagons of
quartz.) Leaving out of view the prevalent strike of the folia in other
parts of this Archipelago, it might have been expected that they would
have dipped N. 28 degrees E., that is directly from the ridge, and,
considering its abruptness, at a high inclination; but the real dip, as
we have just seen, both at the foot and on the northern flank, and over
the entire summit, is at a small angle, and directed nearly due north.
From these considerations it occurred to me, that perhaps we here had
the novel and curious case of already inclined laminae obliquely tilted
at a subsequent period by the granitic axis. Mr. Hopkins, so well known
from his mathematical investigations, has most kindly calculated the
problem: the proposition sent was,—Take a district composed of laminae,
dipping at an angle of 40 degrees to W. 19 degrees S., and let an axis
of elevation traverse it in an E. 28 degrees S. line, what will the
position of the laminae be on the northern flank after a tilt, we will
first suppose, of 45 degrees? Mr. Hopkins informs me, that the angle of
the dip will be 28 degrees 31 minutes, and its direction to north 30
degrees 33 minutes west. (On the south side of the axis (where,
however, I did not see any mica-slate) the dip of the folia would be at
an angle of 77 degrees 55 minutes, directed to west 35 degrees 33
minutes south. Hence the two points of dip on the opposite sides of the
range, instead of being as in ordinary cases directly opposed to each
other at an angle of 180 degrees, would here be only 86 degrees 50
minutes apart.) By varying the supposed angle of the tilt, our
previously inclined folia can be thrown into any angle between 26
degrees, which is the least possible angle, and 90 degrees; but if a
small inclination be thus given to them, their point of dip will depart
far from the north, and therefore not accord with the actual position
of the folia of mica-schist on our granitic range. Hence it appears
very difficult, without varying considerably the elements of the
problem, thus to explain the anomalous strike and dip of the foliated
mica- schist, especially in those parts, namely, at the base of the
range, where the folia are almost horizontal. Mr. Hopkins, however,
adds, that great irregularities and lateral thrusts might be expected
in every great line of elevation, and that these would account for
considerable deviations from the calculated results: considering that
the granitic axis, as shown by the veins, has indisputably been
injected after the perfect formation of the mica-slate, and considering
the uniformity of the strike of the folia throughout the rest of the
Archipelago, I cannot but still think that their anomalous position at
this one point is someway directly and mechanically related to the
intrusion of this W.N.W. and E.S.E. mountain-chain of granite.
Dikes are frequent in the metamorphic schists of the Chonos Islands,
and seem feebly to represent that great band of trappean and ancient
volcanic rocks on the south-western coast of Tierra del Fuego. At S.
Andres I observed in the space of half-a-mile, seven broad, parallel
dikes, composed of three varieties of trap, running in a N.W. and S.E.
line, parallel to the neighbouring mountain-ranges of altered
clay-slate; but they must be of long subsequent origin to these
mountains; for they intersected the volcanic formation described in the
last chapter. North of Tres Montes, I noticed three dikes differing
from each other in composition, one of them having a euritic base
including large octagons of quartz; these dikes, as well as several of
porphyritic greenstone at Vallenar Bay, extended N.E. and S.W., nearly
at right angles to the foliation of the schists, but in the line of
their joints. At Low’s Harbour, however, a set of great parallel dikes,
one ninety yards and another sixty yards in width, have been guided by
the foliation of the mica-schist, and hence are inclined westward at an
angle of 45 degrees: these dikes are formed of various porphyritic
traps, some of which are remarkable from containing numerous rounded
grains of quartz. A porphyritic trap of this latter kind, passed in one
of the dikes into a most curious hornstone, perfectly white, with a
waxy fracture and pellucid edges, fusible, and containing many grains
of quartz and specks of iron pyrites. In the ninety-yard dike several
large, apparently now quite isolated, fragments of mica-slate were
embedded: but as their foliation was exactly parallel to that of the
surrounding solid rock, no doubt these new separate fragments
originally formed wedge-shaped depending portions of a continuous vault
or crust, once extending over the dike, but since worn down and
denuded.
CHILOE, VALDIVIA, CONCEPCION.
In Chiloe, a great formation of mica-schist strikingly resembles that
of the Chonos Islands. For a space of eleven miles on the S.E. coast,
the folia were very distinct, though slightly convoluted, and ranged
within a point of N.N.W. and S.S.E., dipping either E.N.E. or more
commonly W.S.W., at an average angle of 22 degrees (in one spot,
however, at 60 degrees), and therefore decidedly at a lesser
inclination than amongst the Chonos Islands. On the west and
north-western shores, the foliation was often obscure, though, where
best defined, it ranged within a point of N. by W. and S. by E.,
dipping either easterly or westerly, at varying and generally very
small angles. Hence, from the southern part of Tres Montes to the
northern end of Chiloe, a distance of 300 miles, we have closely allied
rocks with their folia striking on an average in the same direction,
namely between N. 11 degrees and 22 degrees W. Again, at Valdivia, we
meet with the same mica-schist, exhibiting nearly the same
mineralogical passages as in the Chonos Archipelago, often, however,
becoming more ferruginous, and containing so much feldspar as to pass
into gneiss. The folia were generally well defined; but nowhere else in
South America did I see them varying so much in direction: this seemed
chiefly caused by their forming parts, as I could sometimes distinctly
trace, of large flat curves: nevertheless, both near the settlement and
towards the interior, a N.W. and S.E. strike seemed more frequent than
any other direction; the angle of the dip was generally small. At
Concepcion, a highly glossy clay-slate had its cleavage often slightly
curvilinear, and inclined, seldom at a high angle, towards various
points of the compass: but here, as at Valdivia, a N.W. and S.E. strike
seemed to be the most frequent one. ((FIGURE 23.) I observed in some
parts that the tops of the laminae of the clay-slate (b in Figure 23)
under the superficial detritus and soil (a) were bent, sometimes
without being broken, as represented in Figure 23, which is copied from
one given by Sir H. De la Beche (page 42 “Geological Manual”) of an
exactly similar phenomenon in Devonshire. Mr. R.A.C. Austen, also, in
his excellent paper on S.E. Devon (“Geological Transactions” volume 6
page 437), has described this phenomenon; he attributes it to the
action of frosts, but at the same time doubts whether the frosts of the
present day penetrate to a sufficient depth. As it is known that
earthquakes particularly affect the surface of the ground, it occurred
to me that this appearance might perhaps be due, at least at
Concepcion, to their frequent occurrence; the superficial layers of
detritus being either jerked in one direction, or, where the surface
was inclined, pushed a little downwards during each strong vibration.
In North Wales I have seen a somewhat analogous but less regular
appearance, though on a greater scale (“London Philosophical Magazine”
volume 21 page 184), and produced by a quite different cause, namely,
by the stranding of great icebergs; this latter appearance has also
been observed in N. America.)
In certain spots large quartz veins were numerous, and near them, the
cleavage, as was the case with the foliation of the schists in the
Chonos Archipelago, became extremely tortuous.
At the northern end of Quiriquina Island, in the Bay of Concepcion, at
least eight rudely parallel dikes, which have been guided to a certain
extent by the cleavage of the slate, occur within the space of a
quarter of a mile. They vary much in composition, resembling in many
respects the dikes at Low’s Harbour: the greater number consist of
feldspathic porphyries, sometimes containing grains of quartz: one,
however, was black and brilliant, like an augitic rock, but really
formed of feldspar; others of a feldspathic nature were perfectly
white, with either an earthy or crystalline fracture, and including
grains and regular octagons of quartz; these white varieties passed
into ordinary greenstones. Although, both here and at Low’s Harbour,
the nature of the rock varied considerably in the same dike, yet I
cannot but think that at these two places and in other parts of the
Chonos group, where the dikes, though close to each other and running
parallel, are of different composition, that they must have been formed
at different periods. In the case of Quiriquina this is a rather
interesting conclusion, for these eight parallel dikes cut through the
metamorphic schists in a N.W. and S.E. line, and since their injection
the overlying cretaceous or tertiary strata have been tilted (whilst
still under the sea) from a N.W. by N. and S.E. by S. line; and again,
during the great earthquake of February 1835, the ground in this
neighbourhood was fissured in N.W. and S.E. lines; and from the manner
in which buildings were thrown down, it was evident that the surface
undulated in this same direction. (“Geological Transactions” volume 6
pages 602 and 617. “Journal of Researches” 2nd edition page 307.)
CENTRAL AND NORTHERN CHILE.
Northward of Concepcion, as far as Copiapo, the shores of the Pacific
consist, with the exception of some small tertiary basins, of gneiss,
mica- schist, altered clay-slate, granite, greenstone and syenite:
hence the coast from Tres Montes to Copiapo, a distance of 1,200 miles,
and I have reason to believe for a much greater space, is almost
similarly constituted.
Near Valparaiso the prevailing rock is gneiss, generally including much
hornblende: concretionary balls formed of feldspar, hornblende and
mica, from two or three feet in diameter, are in very many places
conformably enfolded by the foliated gneiss: veins of quartz and
feldspar, including black schorl and well-crystallised epidote, are
numerous. Epidote likewise occurs in the gneiss in thin layers,
parallel to the foliation of the mass. One large vein of a coarse
granitic character was remarkable from in one part quite changing its
character, and insensibly passing into a blackish porphyry, including
acicular crystals of glassy feldspar and of hornblende: I have never
seen any other such case. (Humboldt “Personal Narrative” volume 4 page
60, has described with much surprise, concretionary balls, with
concentric divisions, composed of partially vitreous feldspar,
hornblende, and garnets, included within great veins of gneiss, which
cut across the mica-slate near Venezuela.)
I shall in the few following remarks on the rocks of Chile allude
exclusively to their foliation and cleavage. In the gneiss round
Valparaiso the strike of the foliation is very variable, but I think
about N. by W. and S. by E. is the commonest direction; this likewise
holds good with the cleavage of the altered feldspathic clay-slates,
occasionally met with on the coast for ninety miles north of
Valparaiso. Some feldspathic slate, alternating with strata of
claystone porphyry in the Bell of Quillota and at Jajuel, and
therefore, perhaps, belonging to a later period than the metamorphic
schists on the coast, cleaved in this same direction. In the Eastern
Cordillera, in the Portillo Pass, there is a grand mass of mica- slate,
foliated in a north and south line, and with a high westerly dip: in
the Uspallata range, clay-slate and grauwacke have a highly inclined,
nearly north and south cleavage, though in some parts the strike is
irregular: in the main or Cumbre range, the direction of the cleavage
in the feldspathic clay-slate is N.W. and S.E.
Between Coquimbo and Guasco there are two considerable formations of
mica- slate, in one of which the rock passed sometimes into common
clay-slate and sometimes into a glossy black variety, very like that in
the Chonos Archipelago. The folia and cleavage of these rocks ranged
between [N. and N.W. by N.] and [S. and S.W. by S.]. Near the Port of
Guasco several varieties of altered clay-slate have a quite irregular
cleavage. Between Guasco and Copiapo, there are some siliceous and
talcaceous slates cleaving in a north and south line, with an easterly
dip of between 60 and 70 degrees: high up, also, the main valley of
Copiapo, there is mica-slate with a high easterly dip. In the whole
space between Valparaiso and Copiapo an easterly dip is much more
common than an opposite or westerly one.
CONCLUDING REMARKS ON CLEAVAGE AND FOLIATION.
In this southern part of the southern hemisphere, we have seen that the
cleavage-laminae range over wide areas with remarkable uniformity,
cutting straight through the planes of stratification, but yet being
parallel in strike to the main axes of elevation, and generally to the
outlines of the coast. (In my paper on the Falkland Islands “Geological
Journal” volume 3 page 267, I have given a curious case on the
authority of Captain Sulivan, R.N., of much folded beds of clay-slate,
in some of which the cleavage is perpendicular to the horizon, and in
others it is perpendicular to each curvature or fold of the bed: this
appears a new case.) The dip, however, is as variable, both in angle
and in direction (that is, sometimes being inclined to the one side and
sometimes to the directly opposite side), as the strike is uniform. In
all these respects there is a close agreement with the facts given by
Professor Sedgwick in his celebrated memoir in the “Geological
Transactions,” and by Sir R.I. Murchison in his various excellent
discussions on this subject. The Falkland Islands, and more especially
Tierra del Fuego, offer striking instances of the lines of cleavage,
the principle axes of elevation, and the outlines of the coast,
gradually changing together their courses. The direction which prevails
throughout Tierra del Fuego and the Falkland Islands, namely, from west
with some northing to east with some southing, is also common to the
several ridges in Northern Patagonia and in the western parts of Banda
Oriental: in this latter province, in the Sierra Tapalguen, and in the
Western Falkland Island, the W. by N., or W.N.W. and E.S.E., ridges,
are crossed at right angles by others ranging N.N.E. and S.S.W.
The fact of the cleavage-laminae in the clay-slate of Tierra del Fuego,
where seen cutting straight through the planes of stratification, and
where consequently there could be no doubt about their nature,
differing slightly in colour, texture, and hardness, appears to me very
interesting. In a thick mass of laminated, feldspathic and altered
clay-slate, interposed between two great strata of porphyritic
conglomerate in Central Chile, and where there could be but little
doubt about the bedding, I observed similar slight differences in
composition, and likewise some distinct thin layers of epidote,
parallel to the highly inclined cleavage of the mass. Again, I
incidentally noticed in North Wales, where glaciers had passed over the
truncated edges of the highly inclined laminae of clay-slate, that the
surface, though smooth, was worn into small parallel undulations,
caused by the competent laminae being of slightly different degrees of
hardness. (“London Philosophical Magazine” volume 21 page 182.) With
reference to the slates of North Wales, Professor Sedgwick describes
the planes of cleavage, as “coated over with chlorite and
semi-crystalline matter, which not only merely define the planes in
question, but strike in parallel flakes through the whole mass of the
rock.” (“Geological Transactions” volume 3 page 471.) In some of those
glossy and hard varieties of clay-slate, which may often be seen
passing into mica-schist, it has appeared to me that the cleavage-
planes were formed of excessively thin, generally slighted convoluted,
folia, composed of microscopically minute scales of mica. From these
several facts, and more especially from the case of the clay-slate in
Tierra del Fuego, it must, I think, be concluded, that the same power
which has impressed on the slate its fissile structure or cleavage has
tended to modify its mineralogical character in parallel planes.
Let us now turn to the foliation of the metamorphic schists, a subject
which has been much less attended to. As in the case of
cleavage-laminae, the folia preserve over very large areas a uniform
strike: thus Humboldt found for a distance of 300 miles in Venezuela,
and indeed over a much larger space, gneiss, granite, mica, and
clay-slate, striking very uniformly N.E. and S.W., and dipping at an
angle of between 60 and 70 degrees to N.W. (“Personal Narrative” volume
6 page 59 et seq.); it would even appear from the facts given in this
chapter, that the metamorphic rocks throughout the north-eastern part
of South America are generally foliated within two points of N.E. and
S.W. Over the eastern parts of Banda Oriental, the foliation strikes
with a high inclination, very uniformly N.N.E. to S.S.W., and over the
western parts, in a W. by N. and E. by S. line. For a space of 300
miles on the shores of the Chonos and Chiloe Islands, we have seen that
the foliation seldom deviates more than a point of the compass from a
N. 19 degrees W. and S. 19 degrees E. strike. As in the case of
cleavage, the angle of the dip in foliated rocks is generally high but
variable, and alternates from one side of the line of strike to the
other side, sometimes being vertical: in the Northern Chonos Islands,
however, the folia are inclined almost always to the west; in nearly
the same manner, the cleavage-laminae in Southern Tierra del Fuego
certainly dip much more frequently to S.S.W. than to the opposite
point. In Eastern Banda Oriental, in parts of Brazil, and in some other
districts, the foliation runs in the same direction with the
mountain-ranges and adjoining coast-lines: amongst the Chonos Islands,
however, this coincidence fails, and I have given my reasons for
suspecting that one granitic axis has burst through and tilted the
already inclined folia of mica-schist: in the case of cleavage, the
coincidence between its strike and that of the main stratification
seems sometimes to fail. (Cases are given by Mr. Jukes in his “Geology
of Newfoundland” page 130.) Foliation and cleavage resemble each other
in the planes winding round concretions, and in becoming tortuous where
veins of quartz abound. (I have seen in Brazil and Chile concretions
thus enfolded by foliated gneiss; and Macculloch “Highlands” volume 1
page 64, has described a similar case. For analogous cases in
clay-slate, see Professor Henslow’s Memoir in “Cambridge Philosophical
Transactions” volume 1 page 379, and Macculloch’s “Classification of
Rocks” page 351. With respect to both foliation and cleavage becoming
tortuous where quartz-veins abound, I have seen instances near Monte
Video, at Concepcion, and in the Chonos Islands. See also Mr.
Greenough’s “Critical Examination” page 78.) On the flanks of the
mountains both in Tierra del Fuego and in other countries, I have
observed that the cleavage-planes frequently dip at a high angle
inwards; and this was long ago observed by Von Buch to be the case in
Norway: this fact is perhaps analogous to the folded, fan-like or
radiating structure in the metamorphic schists of the Alps, in which
the folia in the central crests are vertical and on the two flanks
inclined inwards. (Studer in “Edinburgh New Philosophical Journal”
volume 23 page 144.) Where masses of fissile and foliated rocks
alternate together, the cleavage and foliation, in all cases which I
have seen, are parallel. Where in one district the rocks are fissile,
and in another adjoining district they are foliated, the planes of
cleavage and foliation are likewise generally parallel: this is the
case with the feldspathic homogeneous slates in the southern part of
the Chonos group, compared with the fine foliated mica-schists of the
northern part; so again the clay- slate of the whole eastern side of
Tierra del Fuego cleaves in exactly the same line with the foliated
gneiss and mica-slate of the western coast; other analogous instances
might have been adduced. (I have given a case in Australia. See my
“Volcanic Islands.”)
With respect to the origin of the folia of quartz, mica, feldspar, and
other minerals composing the metamorphic schists, Professor Sedgwick,
Mr. Lyell, and most authors believe, that the constituent parts of each
layer were separately deposited as sediment, and then metamorphosed.
This view, in the majority of cases, I believe to be quite untenable.
In those not uncommon instances, where a mass of clay-slate, in
approaching granite, gradually passes into gneiss, we clearly see that
folia of distinct minerals can originate through the metamorphosis of a
homogeneous fissile rock. (I have described in “Volcanic Islands” a
good instance of such a passage at the Cape of Good Hope.) The
deposition, it may be remarked, of numberless alternations of pure
quartz, and of the elements of mica or feldspar does not appear a
probable event. (See some excellent remarks on this subject, in
D’Aubuisson’s “Traite de Geog.” tome 1 page 297. Also some remarks by
Mr. Dana in “Silliman’s American Journal” volume 45 page 108.) In those
districts in which the metamorphic schists are foliated in planes
parallel to the cleavage of the rocks in an adjoining district, are we
to believe that the folia are due to sedimentary layers, whilst the
cleavage- laminae, though parallel, have no relation whatever to such
planes of deposition? On this view, how can we reconcile the vastness
of the areas over which the strike of the foliation is uniform, with
what we see in disturbed districts composed of true strata: and
especially, how can we understand the high and even vertical dip
throughout many wide districts, which are not mountainous, and
throughout some, as in Western Banda Oriental, which are not even
hilly? Are we to admit that in the northern part of the Chonos
Archipelago, mica-slate was first accumulated in parallel horizontal
folia to a thickness of about four geographical miles, and then
upturned at an angle of forty degrees; whilst, in the southern part of
this same Archipelago, the cleavage-laminae of closely allied rocks,
which none would imagine had ever been horizontal, dip at nearly the
same angle, to nearly the same point?
Seeing, then, that foliated schists indisputably are sometimes produced
by the metamorphosis of homogeneous fissile rocks; seeing that
foliation and cleavage are so closely analogous in the several
above-enumerated respects; seeing that some fissile and almost
homogeneous rocks show incipient mineralogical changes along the planes
of their cleavage, and that other rocks with a fissile structure
alternate with, and pass into varieties with a foliated structure, I
cannot doubt that in most cases foliation and cleavage are parts of the
same process: in cleavage there being only an incipient separation of
the constituent minerals; in foliation a much more complete separation
and crystallisation.
The fact often referred to in this chapter, of the foliation and the
so- called strata in the metamorphic series,—that is, the alternating
masses of different varieties of gneiss, mica-schist, and
hornblende-slate, etc.,—being parallel to each other, at first appears
quite opposed to the view, that the folia have no relation to the
planes of original deposition. Where the so-called beds are not very
thick and of widely different mineralogical composition from each
other, I do not think that there is any difficulty in supposing that
they have originated in an analogous manner with the separate folia. We
should bear in mind what thick strata, in ordinary sedimentary masses,
have obviously been formed by a concretionary process. In a pile of
volcanic rocks on the Island of Ascension, there are strata, differing
quite as much in appearance as the ordinary varieties of the
metamorphic schists, which undoubtedly have been produced, not by
successive flowings of lava, but by internal molecular changes. Near
Monte Video, where the stratification, as it would be called, of the
metamorphic series is, in most parts, particularly well developed,
being as usual, parallel to the foliation, we have seen that a mass of
chloritic schist, netted with quartz-veins, is entangled in gneiss, in
such a manner as to show that it had certainly originated in some
process of segregation: again, in another spot, the gneiss tended to
pass into hornblendic schist by alternating with layers of quartz; but
these layers of quartz almost certainly had never been separately
deposited, for they were absolutely continuous with the numerous
intersecting veins of quartz. I have never had an opportunity of
tracing for any distance, along the line both of strike and of dip, the
so-called beds in the metamorphic schists, but I strongly suspect that
they would not be found to extend with the same character, very far in
the line either of their dip or strike. Hence I am led to believe, that
most of the so-called beds are of the nature of complex folia, and have
not been separately deposited. Of course, this view cannot be extended
to THICK masses included in the metamorphic series, which are of
totally different composition from the adjoining schists, and which are
far extended, as is sometimes the case with quartz and marble; these
must generally be of the nature of true strata. (Macculloch
“Classification of Rocks” page 364, states that primary limestones are
often found in irregular masses or great nodules, “which can scarcely
be said to possess a stratified shape!”) Such strata, however, will
almost always strike in the same direction with the folia, owing to the
axes of elevation being in most countries parallel to the strike of the
foliation; but they will generally dip at a different angle from that
of the foliation; and the angle of the foliation in itself almost
always varies much: hence, in crossing a metamorphosed schistose
district, it would require especial attention to discriminate between
true strata of deposition and complex foliated masses. The mere
presence of true strata in the midst of a set of metamorphic schists,
is no argument that the foliation is of sedimentary origin, without it
be further shown in each case, that the folia not only strike, but dip
throughout in parallel planes with those of the true stratification.
As in some cases it appears that where a fissile rock has been exposed
to partial metamorphic action, for instance from the irruption of
granite, the foliation has supervened on the already existing
cleavage-planes; so perhaps in some instances, the foliation of a rock
may have been determined by the original planes of deposition or of
oblique current-laminae: I have, however, myself, never seen such a
case, and I must maintain that in most extensive metamorphic areas, the
foliation is the extreme result of that process, of which cleavage is
the first effect. That foliation may arise without any previous
structural arrangement in the mass, we may infer from injected, and
therefore once liquified, rocks, both of volcanic and plutonic origin,
sometimes having a “grain” (as expressed by Professor Sedgwick), and
sometimes being composed of distinct folia or laminae of different
compositions. In my work on “Volcanic Islands,” I have given several
instances of this structure in volcanic rocks, and it is not uncommonly
seen in plutonic masses—thus, in the Cordillera of Chile, there are
gigantic mountain-like masses of red granite, which have been injected
whilst liquified, and which, nevertheless, display in parts a decidedly
laminar structure. (As remarked in a former part of this chapter, I
suspect that the boldly conical mountains of gneiss-granite, near Rio
de Janeiro, in which the constituent minerals are arranged in parallel
planes, are of intrusive origin. We must not, however, forget the
lesson of caution taught by the curious claystone porphyries of Port
Desire, in which we have seen that the breaking up and aggregation of a
thinly stratified tufaceous mass, has yielded a rock semi-porphyritic
with crystals of feldspar, arranged in the planes of original
deposition.)
Finally, we have seen that the planes of cleavage and of foliation,
that is, of the incipient process and of the final result, generally
strike parallel to the principal axes of elevation, and to the outline
of the land: the strike of the axes of elevation (that is, of the lines
of fissures with the strata on their edges upturned), according to the
reasoning of Mr. Hopkins, is determined by the form of the area
undergoing changes of level, and the consequent direction of the lines
of tension and fissure. Now, in that remarkable pile of volcanic rocks
at Ascension, which has several times been alluded to (and in some
other cases), I have endeavoured to show, that the lamination of the
several varieties, and their alternations, have been caused by the
moving mass, just before its final consolidation, having been subjected
(as in a glacier) to planes of different tension; this difference in
the tension affecting the crystalline and concretionary processes. (In
“Volcanic Islands.”) One of the varieties of rock thus produced at
Ascension, at first sight, singularly resembles a fine-grained gneiss;
it consists of quite straight and parallel zones of excessive tenuity,
of more or less coloured crystallised feldspar, of distinct crystals of
quartz, diopside, and oxide of iron. These considerations,
notwithstanding the experiments made by Mr. Fox, showing the influence
of electrical currents in producing a structure like that of cleavage,
and notwithstanding the apparently inexplicable variation, both in the
inclination of the cleavage-laminae and in their dipping first to one
side and then to the other side of the line of strike, lead me to
suspect that the planes of cleavage and foliation are intimately
connected with the planes of different tension, to which the area was
long subjected, AFTER the main fissures or axes of upheavement had been
formed, but BEFORE the final consolidation of the mass and the total
cessation of all molecular movement.
CHAPTER VII.
CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.
Central Chile.—Basal formations of the Cordillera.—Origin of the
porphyritic clay-stone conglomerate.—Andesite.—Volcanic rocks.—Section
of the Cordillera by the Peuquenes are Portillo Pass.—Great gypseous
formation.—Peuquenes line; thickness of strata, fossils of.—Portillo
line.—Conglomerate, orthitic granite, mica-schist, volcanic rocks
of.—Concluding remarks on the denudation and elevation of the Portillo
line.—Section by the Cumbre, or Uspallata Pass.—Porphyries.—Gypseous
strata.—Section near the Puente del Inca; fossils of.—Great
subsidence.—Intrusive porphyries.—Plain of Uspallata.—Section of the
Uspallata chain.—Structure and nature of the strata.—Silicified
vertical trees.—Great subsidence.—Granitic rocks of axis.—Concluding
remarks on the Uspallata range; origin subsequent to that of the main
Cordillera; two periods of subsidence; comparison with the Portillo
chain.—
The district between the Cordillera and the Pacific, on a rude average,
is from about eighty to one hundred miles in width. It is crossed by
many chains of mountains, of which the principal ones, in the latitude
of Valparaiso and southward of it, range nearly north and south; but in
the more northern parts of the province, they run in almost every
possible direction. Near the Pacific, the mountain-ranges are generally
formed of syenite or granite, and or of an allied euritic porphyry; in
the low country, besides these granitic rocks and greenstone, and much
gneiss, there are, especially northward of Valparaiso, some
considerable districts of true clay-slate with quartz veins, passing
into a feldspathic and porphyritic slate; there is also some grauwacke
and quartzose and jaspery rocks, the latter occasionally assuming the
character of the basis of claystone porphyry: trap-dikes are numerous.
Nearer the Cordillera the ranges (such as those of S. Fernando, the
Prado (Meyen “Reise um Erde” th. 1 s. 235.), and Aconcagua) are formed
partly of granitic rocks, and partly of purple porphyritic
conglomerates, claystone porphyry, greenstone porphyry, and other
rocks, such as we shall immediately see, form the basal strata of the
main Cordillera. In the more northern parts of Chile, this porphyritic
series extends over large tracts of country far from the Cordillera;
and even in Central Chile such occasionally occur in outlying
positions.
I will describe the Campana of Quillota, which stands only fifteen
miles from the Pacific, as an instance of one of these outlying masses.
This hill is conspicuous from rising to the height of 6,400 feet: its
summit shows a nucleus, uncovered for a height of 800 feet, of fine
greenstone, including epidote and octahedral magnetic iron ore; its
flanks are formed of great strata of porphyritic claystone conglomerate
associated with various true porphyries and amygdaloids, alternating
with thick masses of a highly feldspathic, sometimes porphyritic,
pale-coloured slaty rock, with its cleavage-laminae dipping inwards at
a high angle. At the base of the hill there are syenites, a granular
mixture of quartz and feldspar, and harsh quartzose rocks, all
belonging to the basal metamorphic series. I may observe that at the
foot of several hills of this class, where the porphyries are first
seen (as near S. Fernando, the Prado, Las Vacas, etc.), similar harsh
quartzose rocks and granular mixtures of quartz and feldspar occur, as
if the more fusible constituent parts of the granitic series had been
drawn off to form the overlying porphyries.
In Central Chile, the flanks of the main Cordillera, into which I
penetrated by four different valleys, generally consist of distinctly
stratified rocks. The strata are inclined at angles varying from
sometimes even under ten, to twenty degrees, very rarely exceeding
forty degrees: in some, however, of the quite small, exterior,
spur-like ridges, the inclination was not unfrequently greater. The dip
of the strata in the main outer lines was usually outwards or from the
Cordillera, but in Northern Chile frequently inwards,—that is, their
basset-edges fronted the Pacific. Dikes occur in extraordinary numbers.
In the great, central, loftiest ridges, the strata, as we shall
presently see, are almost always highly inclined and often vertical.
Before giving a detailed account of my two sections across the
Cordillera, it will, I think, be convenient to describe the basal
strata as seen, often to a thickness of four or five thousand feet, on
the flanks of the outer lines.
BASAL STRATA OF THE CORDILLERA.
The prevailing rock is a purplish or greenish, porphyritic claystone
conglomerate. The embedded fragments vary in size from mere particles
to blocks as much as six or eight inches (rarely more) in diameter; in
many places, where the fragments were minute, the signs of aqueous
deposition were unequivocally distinct; where they were large, such
evidence could rarely be detected. The basis is generally porphyritic
with perfect crystals of feldspar, and resembles that of a true
injected claystone porphyry: often, however, it has a mechanical or
sedimentary aspect, and sometimes (as at Jajuel) is jaspery. The
included fragments are either angular, or partially or quite rounded
(Some of the rounded fragments in the porphyritic conglomerate near the
Baths of Cauquenes, were marked with radii and concentric zones of
different shades of colour: any one who did not know that pebbles, for
instance flint pebbles from the chalk, are sometimes zoned
concentrically with their worn and rounded surfaces, might have been
led to infer, that these balls of porphyry were not true pebbles, but
had originated in concretionary action.); in some parts the rounded, in
others the angular fragments prevail, and usually both kinds are mixed
together: hence the word BRECCIA ought strictly to be appended to the
term PORPHYRITIC CONGLOMERATE. The fragments consist of many varieties
of claystone porphyry, usually of nearly the same colour with the
surrounding basis, namely, purplish-reddish, brownish, mottled or
bright green; occasionally fragments of a laminated, pale-coloured,
feldspathic rock, like altered clay-slate are included; as are
sometimes grains of quartz, but only in one instance in Central Chile
(namely, at the mines of Jajuel) a few pebbles of quartz. I nowhere
observed mica in this formation, and rarely hornblende; where the
latter mineral did occur, I was generally in doubt whether the mass
really belonged to this formation, or was of intrusive origin.
Calcareous spar occasionally occurs in small cavities; and nests and
layers of epidote are common. In some few places in the finer-grained
varieties (for instance, at Quillota), there were short, interrupted
layers of earthy feldspar, which could be traced, exactly as at Port
Desire, passing into large crystals of feldspar: I doubt, however,
whether in this instance the layers had ever been separately deposited
as tufaceous sediment.
All the varieties of porphyritic conglomerates and breccias pass into
each other, and by innumerable gradations into porphyries no longer
retaining the least trace of mechanical origin: the transition appears
to have been effected much more easily in the finer-grained, than in
the coarser-grained varieties. In one instance, near Cauquenes, I
noticed that a porphyritic conglomerate assumed a spheroidal structure,
and tended to become columnar. Besides the porphyritic conglomerates
and the perfectly characterised porphyries, of metamorphic origin,
there are other porphyries, which, though differing not at all or only
slightly in composition, certainly have had a different origin: these
consist of pink or purple claystone porphyries, sometimes including
grains of quartz,—of greenstone porphyry, and of other dusky rocks, all
generally porphyritic with fine, large, tabular, opaque crystals, often
placed crosswise, of feldspar cleaving like albite (judging from
several measurements), and often amygdaloidal with silex, agate,
carbonate of lime, green and brown bole. (This bole is a very common
mineral in the amygdaloidal rocks; it is generally of a greenish- brown
colour, with a radiating structure; externally it is black with an
almost metallic lustre, but often coated by a bright green film. It is
soft and can be scratched by a quill; under the blowpipe swells greatly
and becomes scaly, then fuses easily into a black magnetic bead. This
substance is evidently similar to that which often occurs in submarine
volcanic rocks. An examination of some very curious specimens of a fine
porphyry (from Jajuel) leads me to suspect that some of these
amygdaloidal balls, instead of having been deposited in pre-existing
air-vesicles, are of concretionary origin; for in these specimens, some
of the pea-shaped little masses (often externally marked with minute
pits) are formed of a mixture of green earth with stony matter, like
the basis of the porphyry, including minute imperfect crystals of
feldspar; and these pea-shaped little masses are themselves
amygdaloidal with minute spheres of the green earth, each enveloped by
a film of white, apparently feldspathic, earthy matter: so that the
porphyry is doubly amygdaloidal. It should not, however, be overlooked,
that all the strata here have undergone metamorphic action, which may
have caused crystals of feldspar to appear, and other changes to be
effected, in the originally simple amygdaloidal balls. Mr. J.D. Dana,
in an excellent paper on Trap-rocks “Edinburgh New Philosophical
Journal” volume 41 page 198, has argued with great force, that all
amygdaloidal minerals have been deposited by aqueous infiltration. I
may take this opportunity of alluding to a curious case, described in
my work on “Volcanic Islands,” of an amygdaloid with many of its cells
only half filled up with a mesotypic mineral. M. Rose has described an
amygdaloid, brought by Dr. Meyen “Reise um Erde” Th. 1. s. 316, from
Chile, as consisting of crystallised quartz, with crystals of stilbite
within, and lined externally by green earth.) These several porphyritic
and amygdaloidal varieties never show any signs of passing into masses
of sedimentary origin: they occur both in great and small intrusive
masses, and likewise in strata alternating with those of the
porphyritic conglomerate, and with the planes of junction often quite
distinct, yet not seldom blended together. In some of these intrusive
masses, the porphyries exhibit, more or less plainly, a brecciated
structure, like that often seen in volcanic masses. These brecciated
porphyries could generally be distinguished at once from the
metamorphosed, porphyritic breccia- conglomerates, by all the fragments
being angular and being formed of the same variety, and by the absence
of every trace of aqueous deposition. One of the porphyries above
specified, namely, the greenstone porphyry with large tabular crystals
of albite, is particularly abundant, and in some parts of the
Cordillera (as near St. Jago) seemed more common even than the purplish
porphyritic conglomerate. Numerous dikes likewise consist of this
greenstone porphyry; others are formed of various fine-grained trappean
rocks; but very few of claystone porphyry: I saw no true basaltic
dikes.
In several places in the lower part of the series, but not everywhere,
thick masses of a highly feldspathic, often porphyritic, slaty rock
occur interstratified with the porphyritic conglomerate; I believe in
one or two cases blackish limestone has been found in a similar
position. The feldspathic rock is of a pale grey or greenish colour; it
is easily fusible; where porphyritic, the crystals of feldspar are
generally small and vitreous: it is distinctly laminated, and sometimes
includes parallel layers of epidote (This mineral is extremely common
in all the formations of Chile; in the gneiss near Valparaiso and in
the granitic veins crossing it, in the injected greenstone crowning the
C. of Quillota, in some granitic porphyries, in the porphyritic
conglomerate, and in the feldspathic clay-slates.); the lamination
appears to be distinct from stratification. Occasionally this rock is
somewhat curious; and at one spot, namely, at the C. of Quillota, it
had a brecciated structure. Near the mines of Jajuel, in a thick
stratum of this feldspathic, porphyritic slate, there was a layer of
hard, blackish, siliceous, infusible, compact clay-slate, such as I saw
nowhere else; at the same place I was able to follow for a considerable
distance the junction between the slate and the conformably underlying
porphyritic conglomerate, and they certainly passed gradually into each
other. Wherever these slaty feldspathic rocks abound, greenstone seems
common; at the C. of Quillota a bed of well-crystallised greenstone lay
conformably in the midst of the feldspathic slate, with the upper and
lower junctions passing insensibly into it. From this point, and from
the frequently porphyritic condition of the slate, I should perhaps
have considered this rock as an erupted one (like certain laminated
feldspathic lavas in the trachytic series), had I not seen in Tierra
del Fuego how readily true clay-slate becomes feldspathic and
porphyritic, and had I not seen at Jajuel the included layer of black,
siliceous clay-slate, which no one could have thought of igneous
origin. The gentle passage of the feldspathic slate, at Jajuel, into
the porphyritic conglomerate, which is certainly of aqueous origin,
should also be taken in account.
The alternating strata of porphyries and porphyritic conglomerate, and
with the occasionally included beds of feldspathic slate, together make
a grand formation; in several places within the Cordillera, I estimated
its thickness at from six to seven thousand feet. It extends for many
hundred miles, forming the western flank of the Chilean Cordillera; and
even at Iquique in Peru, 850 miles north of the southernmost point
examined by me in Chile, the coast-escarpment which rises to a height
of between two and three thousand feet is thus composed. In several
parts of Northern Chile this formation extends much further towards the
Pacific, over the granitic and metamorphic lower rocks, than it does in
Central Chile; but the main Cordillera may be considered as its central
line, and its breadth in an east and west direction is never great. At
first the origin of this thick, massive, long but narrow formation,
appeared to me very anomalous: whence were derived, and how were
dispersed the innumerable fragments, often of large size, sometimes
angular and sometimes rounded, and almost invariably composed of
porphyritic rocks? Seeing that the interstratified porphyries are never
vesicular and often not even amygdaloidal, we must conclude that the
pile was formed in deep water; how then came so many fragments to be
well rounded and so many to remain angular, sometimes the two kinds
being equally mingled, sometimes one and sometimes the other
preponderating? That the claystone, greenstone, and other porphyries
and amygdaloids, which lie CONFORMABLY between the beds of
conglomerate, are ancient submarine lavas, I think there can be no
doubt; and I believe we must look to the craters whence these streams
were erupted, as the source of the breccia- conglomerate; after the
great explosion, we may fairly imagine that the water in the heated and
scarcely quiescent crater would remain for a considerable time
sufficiently agitated to triturate and round the loose fragments, few
or many in number, would be shot forth at the next eruption, associated
with few or many angular fragments, according to the strength of the
explosion. (This certainly seems to have taken place in some recent
volcanic archipelagos, as at the Galapagos, where numerous craters are
exclusively formed of tuff and fragments of lava.) The porphyritic
conglomerate being purple or reddish, even when alternating with dusty-
coloured or bright green porphyries and amygdaloids, is probably an
analogous circumstance to the scoriae of the blackish basalts being
often bright red. The ancient submarine orifices whence the porphyries
and their fragments were ejected having been arranged in a band, like
most still active volcanoes, accounts for the thickness, the
narrowness, and linear extension of this formation.
This whole great pile of rock has suffered much metamorphic action, as
is very obvious in the gradual formation and appearance of the crystals
of albitic feldspar and of epidote—in the bending together of the
fragments— in the appearance of a laminated structure in the
feldspathic slate—and, lastly, in the disappearance of the planes of
stratification, which could sometimes be seen on the same mountain
quite distinct in the upper part, less and less plain on the flanks,
and quite obliterated at the base. Partly owing to this metamorphic
action, and partly to the close relationship in origin, I have seen
fragments of porphyries—taken from a metamorphosed conglomerate—from a
neighbouring stream of lava—from the nucleus or centre (as it appeared
to me) of the whole submarine volcano— and lastly from an intrusive
mass of quite subsequent origin, all of which were absolutely
undistinguishable in external characters.
One other rock, of plutonic origin, and highly important in the history
of the Cordillera, from having been injected in most of the great axes
of elevation, and from having apparently been instrumental in
metamorphosing the superincumbent strata, may be conveniently described
in this preliminary discussion. It has been called by some authors
ANDESITE: it mainly consists of well-crystallised white albite (as
determined with the goniometer in numerous specimens both by Professor
Miller and myself), of less perfectly crystallised green hornblende,
often associated with much mica, with chlorite and epidote, and
occasionally with a few grains of quartz: in one instance in Northern
Chile, I found crystals of orthitic or potash feldspar, mingled with
those of albite. (I here, and elsewhere, call by this name, those
feldspathic minerals which cleave like albite: but it now appears
(“Edinburgh New Philosophical Journal” volume 24 page 181) that Abich
has analysed a mineral from the Cordillera, associated with hornblende
and quartz (probably the same rock with that here under discussion),
which cleaves like albite, but which is a new and distinct kind, called
by him ANDESINE. It is allied to leucite, with the greater proportion
of its potash replaced by lime and soda. This mineral seems scarcely
distinguishable from albite, except by analysis.) Where the mica and
quartz are abundant, the rock cannot be distinguished from granite; and
it may be called andesitic granite. Where these two minerals are quite
absent, and when, as often then happens, the crystals of albite are
imperfect and blend together, the rock may be called andesitic
porphyry, which bears nearly the same relation to andesitic granite
that euritic porphyry does to common granite. These andesitic rocks
form mountain masses of a white colour, which, in their general outline
and appearance—in their joints—in their occasionally including
dark-coloured, angular fragments, apparently of some pre-existing
rock—and in the great dikes branching from them into the superincumbent
strata, manifest a close and striking resemblance to masses of common
granite and syenite: I never, however, saw in these andesitic rocks,
those granitic veins of segregation which are so common in true
granites. We have seen that andesite occurs in three places in Tierra
del Fuego; in Chile, from S. Fernando to Copiapo, a distance of 450
miles, I found it under most of the axes of elevation; in a collection
of specimens from the Cordillera of Lima in Peru, I immediately
recognised it; and Erman states that it occurs in Eastern Kamtschatka.
(“Geographical Journal” volume 9 page 510.) From its wide range, and
from the important part it has played in the history of the Cordillera,
I think this rock has well deserved its distinct name of Andesite.
The few still active volcanoes in Chile are confined to the central and
loftiest ranges of the Cordillera; and volcanic matter, such as appears
to have been of subaerial eruption, is everywhere rare. According to
Meyen, there is a hill of pumice high up the valley of the Maypu, and
likewise a trachytic formation at Colina, a village situated north of
St. Jago. (“Reise um Erde” Th. 1 ss. 338 and 362.) Close to this latter
city, there are two hills formed of a pale feldspathic porphyry,
remarkable from being doubly columnar, great cylindrical columns being
subdivided into smaller four- or five-sided ones; and a third hillock
(Cerro Blanco) is formed of a fragmentary mass of rock, which I
believed to be of volcanic origin, intermediate in character between
the above feldspathic porphyry and common trachyte, and containing
needles of hornblende and granular oxide of iron. Near the Baths of
Cauquenes, between two short parallel lines of elevation, where they
are intersected by the valley, there is a small, though distinct
volcanic district; the rock is a dark grey (andesitic) trachyte, which
fuses into a greenish-grey bead, and is formed of long crystals of
fractured glassy albite (judging from one measurement) mingled with
well- formed crystals, often twin, of augite. The whole mass is
vesicular, but the surface is darker coloured and much more vesicular
than any other part. This trachyte forms a cliff-bounded, horizontal,
narrow strip on the steep southern side of the valley, at the height of
four or five hundred feet above the river-bed; judging from an
apparently corresponding line of cliff on the northern side, the valley
must once have been filled up to this height by a field of lava. On the
summit of a lofty mountain some leagues higher up this same valley of
the Cachapual, I found columnar pitchstone porphyritic with feldspar; I
do not suppose this rock to be of volcanic origin, and only mention it
here, from its being intersected by masses and dikes of a VESICULAR
rock, approaching in character to trachyte; in no other part of Chile
did I observe vesicular or amygdaloidal dikes, though these are so
common in ordinary volcanic districts.
PASSAGE OF THE ANDES BY THE PORTILLO OR PEQUENES PASS.
Although I crossed the Cordillera only once by this pass, and only once
by that of the Cumbre or Uspallata (presently to be described), riding
slowly and halting occasionally to ascend the mountains, there are many
circumstances favourable to obtaining a more faithful sketch of their
structure than would at first be thought possible from so short an
examination. The mountains are steep and absolutely bare of vegetation;
the atmosphere is resplendently clear; the stratification distinct; and
the rocks brightly and variously coloured: some of the natural sections
might be truly compared for distinctness to those coloured ones in
geological works. Considering how little is known of the structure of
this gigantic range, to which I particularly attended, most travellers
having collected only specimens of the rocks, I think my
sketch-sections, though necessarily imperfect, possess some interest.
Section 1/1 in Plate 1 which I will now describe in detail, is on a
horizontal scale of a third of an inch to a nautical mile, and on a
vertical scale of one inch to a mile (or 6,000 feet). The width of the
range (excluding a few outlying hillocks), from the plain on which St.
Jago the capital of Chile stands, to the Pampas, is sixty miles, as far
as I can judge from the maps, which differ from each other and are all
EXCEEDINGLY imperfect. The St. Jago plain at the mouth of the Maypu, I
estimate from adjoining known points at 2,300 feet, and the Pampas at
3,500 feet, both above the level of the sea. The height of the Pequenes
line, according to Dr. Gillies, is 13,210 feet (“Journal of Natural and
Geographical Science” August 1830.); and that of the Portillo line
(both in the gaps where the road crosses them) is 14,345 feet; the
lowest part of the intermediate valley of Tenuyan is 7,530 feet—all
above the level of the sea.
The Cordillera here, and indeed I believe throughout Chile, consist of
several parallel, anticlinal and uniclinal mountain-lines, ranging
north, or north with a little westing, and south. Some exterior and
much lower ridges often vary considerably from this course, projecting
like oblique spurs from the main ranges: in the district towards the
Pacific, the mountains, as before remarked, extend in various
directions, even east and west. In the main exterior lines, the strata,
as also before remarked, are seldom inclined at a high angle; but in
the central lofty ridges they are almost always highly inclined, broken
by many great faults, and often vertical. As far as I could judge, few
of the ranges are of great length: and in the central parts of the
Cordillera, I was frequently able to follow with my eye a ridge
gradually becoming higher and higher, as the stratification increased
in inclination, from one end where its height was trifling and its
strata gently inclined to the other end where vertical strata formed
snow-clad pinnacles. Even outside the main Cordillera, near the baths
of Cauquenes, I observed one such case, where a north and south ridge
had its strata in the valley inclined at 37 degrees, and less than a
mile south of it at 67 degrees: another parallel and similarly inclined
ridge rose at the distance of about five miles, into a lofty mountain
with absolutely vertical strata. Within the Cordillera, the height of
the ridges and the inclination of the strata often became doubled and
trebled in much shorter distances than five miles; this peculiar form
of upheaval probably indicates that the stratified crust was thin, and
hence yielded to the underlying intrusive masses unequally, at certain
points on the lines of fissure.
The valleys, by which the Cordillera are drained, follow the anticlinal
or rarely synclinal troughs, which deviate most from the usual north
and south course; or still more commonly those lines of faults or of
unequal curvature (that is, lines with the strata on both hands dipping
in the same direction, but at a somewhat different angle) which deviate
most from a northerly course. Occasionally the torrents run for some
distance in the north and south valleys, and then recover their eastern
or western course by bursting through the ranges at those points where
the strata have been least inclined and the height consequently is
less. Hence the valleys, along which the roads run, are generally
zigzag; and, in drawing an east and west section, it is necessary to
contract greatly that which is actually seen on the road.
Commencing at the western end of Section 1/1 where the R. Maypu
debouches on the plain of St. Jago, we immediately enter on the
porphyritic conglomerate formation, and in the midst of it find some
hummocks [A] of granite and syenite, which probably (for I neglected to
collect specimens) belong to the andesitic class. These are succeeded
by some rugged hills [B] of dark-green, crystalline, feldspathic and in
some parts slaty rocks, which I believe belong to the altered
clay-slate formation. From this point, great mountains of purplish and
greenish, generally thinly stratified, highly porphyritic
conglomerates, including many strata of amygdaloidal and greenstone
porphyries, extend up the valley to the junction of the rivers Yeso and
Volcan. As the valley here runs in a very southerly course, the width
of the porphyritic conglomerate formation is quite conjectural; and
from the same cause, I was unable to make out much about the
stratification. In most of the exterior mountains the dip was gentle
and directed inwards; and at only one spot I observed an inclination as
high as 50 degrees. Near the junction of the R. Colorado with the main
stream, there is a hill of whitish, brecciated, partially decomposed
feldspathic porphyry, having a volcanic aspect but not being really of
that nature: at Tolla, however, in this valley, Dr. Meyen met with a
hill of pumice containing mica. (“Reise um Erde” Th.1 ss. 338, 341.) At
the junction of the Yeso and Volcan [D] there is an extensive mass, in
white conical hillocks, of andesite, containing some mica, and passing
either into andesitic granite, or into a spotted, semi-granular mixture
of albitic (?) feldspar and hornblende: in the midst of this formation
Dr. Meyen found true trachyte. The andesite is covered by strata of
dark-coloured, crystalline, obscurely porphyritic rocks, and above them
by the ordinary porphyritic conglomerates,—the strata all dipping away
at a small angle from the underlying mass. The surrounding lofty
mountains appear to be entirely composed of the porphyritic
conglomerate, and I estimated its thickness here at between six and
seven thousand feet. Beyond the junction of the Yeso and Volcan, the
porphyritic strata appear to dip towards the hillocks of andesite at an
angle of 40 degrees; but at some distant points on the same ridge they
are bent up and vertical. Following the valley of the Yeso, trending
N.E. (and therefore still unfavourable for our transverse section), the
same porphyritic conglomerate formation is prolonged to near the
Cuestadel Indio, situated at the western end of the basin (like a
drained lake) of Yeso. Some way before arriving at this point, distant
lofty pinnacles capped by coloured strata belonging to the great
gypseous formation could first be seen. From the summit of the Cuesta,
looking southward, there is a magnificent sectional view of a
mountain-mass, at least 2,000 feet in thickness [E], of fine andesite
granite (containing much black mica, a little chlorite and quartz),
which sends great white dikes far into the superincumbent,
dark-coloured, porphyritic conglomerates. At the line of junction the
two formations are wonderfully interlaced together: in the lower part
of the porphyritic conglomerate, the stratification has been quite
obliterated, whilst in the upper part it is very distinct, the beds
composing the crests of the surrounding mountains being inclined at
angles of between 70 and 80 degrees, and some being even vertical. On
the northern side of the valley, there is a great corresponding mass of
andesitic granite, which is encased by porphyritic conglomerate,
dipping both on the western and eastern sides, at about 80 degrees to
west, but on the eastern side with the tips of the strata bent in such
a manner, as to render it probable that the whole mass has been on that
side thrown over and inverted.
In the valley basin of the Yeso, which I estimated at 7,000 feet above
the level of the sea, we first reach at [F] the gypseous formation. Its
thickness is very great. It consists in most parts of snow-white, hard,
compact gypsum, which breaks with a saccharine fracture, having
translucent edges; under the blowpipe gives out much vapour; it
frequently includes nests and exceedingly thin layers of crystallised,
blackish carbonate of lime. Large, irregularly shaped concretions
(externally still exhibiting lines of aqueous deposition) of
blackish-grey, but sometimes white, coarsely and brilliantly
crystallised, hard anhydrite, abound within the common gypsum.
Hillocks, formed of the hardest and purest varieties of the white
gypsum, stand up above the surrounding parts, and have their surfaces
cracked and marked, just like newly baked bread. There is much pale
brown, soft argillaceous gypsum; and there were some intercalated green
beds which I had not time to reach. I saw only one fragment of selenite
or transparent gypsum, and that perhaps may have come from some
subsequently formed vein. From the mineralogical characters here given,
it is probable that these gypseous beds have undergone some metamorphic
action. The strata are much hidden by detritus, but they appeared in
most parts to be highly inclined; and in an adjoining lofty pinnacle
they could be distinctly seen bending up, and becoming vertical,
conformably with the underlying porphyritic conglomerate. In very many
parts of the great mountain-face [F], composed of thin gypseous beds,
there were innumerable masses, irregularly shaped and not like dikes,
yet with well-defined edges, of an imperfectly granular, pale greenish,
or yellowish-white rock, essentially composed of feldspar, with a
little chlorite or hornblende, epidote, iron-pyrites, and ferruginous
powder: I believe that these curious trappean masses have been injected
from the not far distant mountain-mass [E] of andesite whilst still
fluid, and that owing to the softness of the gypseous strata they have
not acquired the ordinary forms of dikes. Subsequently to the injection
of these feldspathic rocks, a great dislocation has taken place; and
the much shattered gypseous strata here overlie a hillock [G], composed
of vertical strata of impure limestone and of black highly calcareous
shale including threads of gypsum: these rocks, as we shall presently
see, belong to the upper parts of the gypseous series, and hence must
here have been thrown down by a vast fault.
Proceeding up the valley-basin of the Yeso, and taking our section
sometimes on one hand and sometimes on the other, we come to a great
hill of stratified porphyritic conglomerate [H] dipping at 45 degrees
to the west; and a few hundred yards farther on, we have a bed between
three or four hundred feet thick of gypsum [I] dipping eastward at a
very high angle: here then we have a fault and anticlinal axis. On the
opposite side of the valley, a vertical mass of red conglomerate,
conformably underlying the gypsum, appears gradually to lose its
stratification and passes into a mountain of porphyry. The gypsum [I]
is covered by a bed [K], at least 1,000 feet in thickness, of a
purplish-red, compact, heavy, fine-grained sandstone or mudstone, which
fuses easily into a white enamel, and is seen under a lens to contain
triturated crystals. This is succeeded by a bed [L], 1,000 feet thick
(I believe I understate the thickness) of gypsum, exactly like the beds
before described; and this again is capped by another great bed [M] of
purplish-red sandstone. All these strata dip eastward; but the
inclination becomes less and less, as we leave the first and almost
vertical bed [I] of gypsum.
Leaving the basin-plain of Yeso, the road rapidly ascends, passing by
mountains composed of the gypseous and associated beds, with their
stratification greatly disturbed and therefore not easily intelligible:
hence this part of the section has been left uncoloured. Shortly before
reaching the great Pequenes ridge, the lowest stratum visible [N] is a
red sandstone or mudstone, capped by a vast thickness of black,
compact, calcareous, shaly rock [O], which has been thrown into four
lofty, though small ridges: looking northward, the strata in these
ridges are seen gradually to rise in inclination, becoming in some
distant pinnacles absolutely vertical.
The ridge of Pequenes, which divides the waters flowing into the
Pacific and Atlantic Oceans, extends in a nearly N.N.W. and S.S.E.
line; its strata dip eastward at an angle of between 30 and 45 degrees,
but in the higher peaks bending up and becoming almost vertical. Where
the road crosses this range, the height is 13,210 feet above the
sea-level, and I estimated the neighbouring pinnacles at from fourteen
to fifteen thousand feet. The lowest stratum visible in this ridge is a
red stratified sandstone [P]; on it are superimposed two great masses
[Q and S] of black, hard, compact, even having a conchoidal fracture,
calcareous, more or less laminated shale, passing into limestone: this
rock contains organic remains, presently to be enumerated. The
compacter varieties fuse easily in a white glass; and this I may add is
a very general character with all the sedimentary beds in the
Cordillera: although this rock when broken is generally quite black, it
everywhere weathers into an ash-grey tint. Between these two great
masses [Q and S], a bed [R] of gypsum is interposed, about three
hundred feet in thickness, and having the same characters as heretofore
described. I estimated the total thickness of these three beds [Q, R,
S] at nearly three thousand feet; and to this must be added, as will be
immediately seen, a great overlying mass of red sandstone.
In descending the eastern slope of this great central range, the
strata, which in the upper part dip eastward at about an angle of 40
degrees, become more and more curved, till they are nearly vertical;
and a little further onwards there is seen on the further side of a
ravine, a thick mass of strata of bright red sandstone [T], with their
upper extremities slightly curved, showing that they were once
conformably prolonged over the beds [S]: on the southern and opposite
side of the road, this red sandstone and the underlying black shaly
rocks stand vertical, and in actual juxtaposition. Continuing to
descend, we come to a synclinal valley filled with rubbish, beyond
which we have the red sandstone [T2] corresponding with [T], and now
dipping, as is seen both north and south of the road, at 45 degrees to
the west; and under it, the beds [S2, R2, Q2, and I believe P2] in
corresponding order and of similar composition, with those on the
western flank of the Pequenes range, but dipping westward. Close to the
synclinal valley the dip of these strata is 45 degrees, but at the
eastern or farther end of the series it increases to 60 degrees. Here
the great gypseous formation abruptly terminates, and is succeeded
eastward by a pile of more modern strata. Considering how violently
these central ranges have been dislocated, and how very numerous dikes
are in the exterior and lower parts of the Cordillera, it is remarkable
that I did not here notice a single dike. The prevailing rock in this
neighbourhood is the black, calcareous, compact shale, whilst in the
valley-basin of the Yeso the purplish red sandstone or mudstone
predominates,—both being associated with gypseous strata of exactly the
same nature. It would be very difficult to ascertain the relative
superposition of these several masses, for we shall afterwards see in
the Cumbre Pass that the gypseous and intercalated beds are
lens-shaped, and that they thin out, even where very thick, and
disappear in short horizontal distances: it is quite possible that the
black shales and red sandstones may be contemporaneous, but it is more
probable that the former compose the uppermost parts of the series.
The fossils above alluded to in the black calcareous shales are few in
number, and are in an imperfect condition; they consist, as named for
me by M. d’Orbigny, of:—
1. Ammonite, indeterminable, near to A. recticostatus, d’Orbigny, “Pal.
Franc.” (Neocomian formation). 2. Gryphaea, near to G. Couloni
(Neocomian formations of France and Neufchatel). 3. Natica,
indeterminable. 4. Cyprina rostrata, d’Orbigny, “Pal. Franc.”
(Neocomian formation). 5. Rostellaria angulosa (?), d’Orbigny, “Pal. de
l’Amer. Mer.” 6. Terebratula (?).
Some of the fragments of Ammonites were as thick as a man’s arm: the
Gryphaea is much the most abundant shell. These fossils M. d’Orbigny
considers as belonging to the Neocomian stage of the Cretaceous system.
Dr. Meyen, who ascended the valley of the Rio Volcan, a branch of the
Yeso, found a nearly similar, but apparently more calcareous formation,
with much gypsum, and no doubt the equivalent of that here described
(“Reise um Erde” etc. Th. 1 s. 355.): the beds were vertical, and were
prolonged up to the limits of perpetual snow; at the height of 9,000
feet above the sea, they abounded with fossils, consisting, according
to Von Buch (“Descript. Phys. des Iles Canaries” page 471.), of:—
1. Exogyra (Gryphaea) Couloni, absolutely identical with specimens from
the Jura and South of France. 2. Trigonia costata, identical with those
found in the upper Jurassic beds at Hildesheim. 3. Pecten striatus,
identical with those found in the upper Jurassic beds at Hildesheim. 4.
Cucullaea, corresponding in form to C. longirostris, so frequent in the
upper Jurassic beds of Westphalia. 5. Ammonites resembling A. biplex.
Von Buch concludes that this formation is intermediate between the
limestone of the Jura and the chalk, and that it is analogous with the
uppermost Jurassic beds forming the plains of Switzerland. Hence M.
D’Orbigny and Von Buch, under different terms, compare these fossils to
those from the same late stage in the secondary formations of Europe.
Some of the fossils which I collected were found a good way down the
western slope of the main ridge, and hence must originally have been
covered up by a great thickness of the black shaly rock, independently
of the now denuded, thick, overlying masses of red sandstone. I
neglected at the time to estimate how many hundred or rather thousand
feet thick the superincumbent strata must have been: and I will not now
attempt to do so. This, however, would have been a highly interesting
point, as indicative of a great amount of subsidence, of which we shall
hereafter find in other parts of the Cordillera analogous evidence
during this same period. The altitude of the Peuquenes Range,
considering its not great antiquity, is very remarkable; many of the
fossils were embedded at the height of 13,210 feet, and the same beds
are prolonged up to at least from fourteen to fifteen thousand feet
above the level of the sea.
THE PORTILLO OR EASTERN CHAIN.
The valley of Tenuyan, separating the Peuquenes and Portillo lines, is,
as estimated by Dr. Gillies and myself, about twenty miles in width;
the lowest part, where the road crosses the river, being 7,500 feet
above the sea-level. The pass on the Portillo line is 14,365 feet high
(1,100 feet higher than that on the Peuquenes), and the neighbouring
pinnacles must, I conceive, rise to nearly 16,000 feet above the sea.
The river draining the intermediate valley of Tenuyan, passes through
the Portillo line. To return to our section:—shortly after leaving the
lower beds [P2] of the gypseous formation, we come to grand masses of a
coarse, red conglomerate [V], totally unlike any strata hitherto seen
in the Cordillera. This conglomerate is distinctly stratified, some of
the beds being well defined by the greater size of the pebbles: the
cement is calcareous and sometimes crystalline, though the mass shows
no signs of having been metamorphosed. The included pebbles are either
perfectly or only partially rounded: they consist of purplish
sandstones, of various porphyries, of brownish limestone, of black
calcareous, compact shale precisely like that in situ in the Peuquenes
range, and CONTAINING SOME OF THE SAME FOSSIL SHELLS; also very many
pebbles of quartz, some of micaceous schist, and numerous, broken,
rounded crystals of a reddish orthitic or potash feldspar (as
determined by Professor Miller), and these from their size must have
been derived from a coarse-grained rock, probably granite. From this
feldspar being orthitic, and even from its external appearance, I
venture positively to affirm that it has not been derived from the
rocks of the western ranges; but, on the other hand, it may well have
come, together with the quartz and metamorphic schists, from the
eastern or Portillo line, for this line mainly consists of coarse
orthitic granite. The pebbles of the fossiliferous slate and of the
purple sandstone, certainly have been derived from the Peuquenes or
western ranges.
The road crosses the valley of Tenuyan in a nearly east and west line,
and for several miles we have on both hands the conglomerate,
everywhere dipping west and forming separate great mountains. The
strata, where first met with, after leaving the gypseous formation, are
inclined westward at an angle of only 20 degrees, which further on
increases to about 45 degrees. The gypseous strata, as we have seen,
are also inclined westward: hence, when looking from the eastern side
of the valley towards the Peuquenes range, a most deceptive appearance
is presented, as if the newer beds of conglomerate dipped directly
under the much older beds of the gypseous formation. In the middle of
the valley, a bold mountain of unstratified lilac-coloured porphyry
(with crystals of hornblende) projects; and further on, a little south
of the road, there is another mountain, with its strata inclined at a
small angle eastwards, which in its general aspect and colour,
resembles the porphyritic conglomerate formation, so rare on this side
of the Peuquenes line and so grandly developed throughout the western
ranges.
The conglomerate is of great thickness: I do not suppose that the
strata forming the separate mountain-masses [V,V,V] have ever been
prolonged over each other, but that one mass has been broken up by
several, distinct, parallel, uniclinal lines of elevation. Judging
therefore of the thickness of the conglomerate, as seen in the separate
mountain-masses, I estimated it at least from one thousand five hundred
to two thousand feet. The lower beds rest conformably on some
singularly coloured, soft strata [W], which I could not reach to
examine; and these again rest conformably on a thick mass of micaceous,
thinly laminated, siliceous sandstone [X], associated with a little
black clay-slate. These lower beds are traversed by several dikes of
decomposing porphyry. The laminated sandstone is directly superimposed
on the vast masses of granite [Y,Y] which mainly compose the Portillo
range. The line of junction between this latter rock, which is of a
bright red colour, and the whitish sandstone was beautifully distinct;
the sandstone being penetrated by numerous, great, tortuous dikes
branching from the granite, and having been converted into a granular
quartz rock (singularly like that of the Falkland Islands), containing
specks of an ochrey powder, and black crystalline atoms, apparently of
imperfect mica. The quartzose strata in one spot were folded into a
regular dome.
The granite which composes the magnificent bare pinnacles and the steep
western flank of the Portillo chain, is of a brick-red colour, coarsely
crystallised, and composed of orthitic or potash feldspar, quartz, and
imperfect mica in small quantity, sometimes passing into chlorite.
These minerals occasionally assume a laminar or foliated arrangement.
The fact of the feldspar being orthitic in this range, is very
remarkable, considering how rare, or rather, as I believe, entirely
absent, this mineral is throughout the western ranges, in which
soda-feldspar, or at least a variety cleaving like albite, is so
extremely abundant. In one spot on the western flank, and on the
eastern flank near Los Manantiales and near the crest, I noticed some
great masses of a whitish granite, parts of it fine- grained, and parts
containing large crystals of feldspar; I neglected to collect
specimens, so I do not know whether this feldspar is also orthitic,
though I am inclined to think so from its general appearance. I saw
also some syenite and one mass which resembled andesite, but of which I
likewise neglected to collect specimens. From the manner in which the
whitish granites formed separate mountain-masses in the midst of the
brick-red variety, and from one such mass near the crest being
traversed by numerous veins of flesh-coloured and greenish eurite (into
which I occasionally observed the brick-red granite insensibly
passing), I conclude that the white granites probably belong to an
older formation, almost overwhelmed and penetrated by the red granite.
On the crest I saw also, at a short distance, some coloured stratified
beds, apparently like those [W] at the western base, but was prevented
examining them by a snowstorm: Mr. Caldcleugh, however, collected here
specimens of ribboned jasper, magnesian limestone, and other minerals.
(“Travels” etc. volume 1 page 308.) A little way down the eastern slope
a few fragments of quartz and mica-slate are met with; but the great
formation of this latter rock [Z], which covers up much of the eastern
flank and base of the Portillo range, cannot be conveniently examined
until much lower down at a place called Mal Paso. The mica-schist here
consists of thick layers of quartz, with intervening folia of
finely-scaly mica, often passing into a substance like black glossy
clay-slate: in one spot, the layers of the quartz having disappeared,
the whole mass became converted into glossy clay-slate. Where the folia
were best defined, they were inclined at a high angle westward, that
is, towards the range. The line of junction between the dark mica-slate
and the coarse red granite was most clearly distinguishable from a vast
distance: the granite sent many small veins into the mica-slate, and
included some angular fragments of it. As the sandstone on the western
base has been converted by the red granite into a granular quartz-rock,
so this great formation of mica-schist may possibly have been
metamorphosed at the same time and by the same means; but I think it
more probable, considering its more perfect metamorphic character and
its well-pronounced foliation, that it belongs to an anterior epoch,
connected with the white granites: I am the more inclined to this view,
from having found at the foot of the range the mica-schist surrounding
a hummock [Y2], exclusively composed of white granite. Near Los
Arenales, the mountains on all sides are composed of the mica-slate;
and looking backwards from this point up to the bare gigantic peaks
above, the view was eminently interesting. The colours of the red
granite and the black mica-slate are so distinct, that with a bright
light these rocks could be readily distinguished even from the Pampas,
at a level of at least 9,000 feet below. The red granite, from being
divided by parallel joints, has weathered into sharp pinnacles, on some
of which, even on some of the loftiest, little caps of mica-schist
could be clearly seen: here and there isolated patches of this rock
adhered to the mountain-flanks, and these often corresponded in height
and position on the opposite sides of the immense valleys. Lower down
the schist prevailed more and more, with only a few quite small points
of granite projecting through. Looking at the entire eastern face of
the Portillo range, the red colour far exceeds in area the black; yet
it was scarcely possible to doubt that the granite had once been almost
wholly encased by the mica-schist.
At Los Arenales, low down on the eastern flank, the mica-slate is
traversed by several closely adjoining, broad dikes, parallel to each
other and to the foliation of the schist. The dikes are formed of three
different varieties of rock, of which a pale brown feldspathic porphyry
with grains of quartz was much the most abundant. These dikes with
their granules of quartz, as well as the mica-schist itself, strikingly
resemble the rocks of the Chonos Archipelago. At a height of about
twelve hundred feet above the dikes, and perhaps connected with them,
there is a range of cliffs formed of successive lava-streams [AA],
between three and four hundred feet in thickness, and in places finely
columnar. The lava consists of dark- greyish, harsh rocks, intermediate
in character between trachyte and basalt, containing glassy feldspar,
olivine, and a little mica, and sometimes amygdaloidal with zeolite:
the basis is either quite compact, or crenulated with air-vesicles
arranged in laminae. The streams are separated from each other by beds
of fragmentary brown scoriae, firmly cemented together, and including a
few well-rounded pebbles of lava. From their general appearance, I
suspect that these lava-streams flowed at an ancient period under the
pressure of the sea, when the Atlantic covered the Pampas and washed
the eastern foot of the Cordillera. (This conclusion might, perhaps,
even have been anticipated, from the general rarity of volcanic action,
except near the sea or large bodies of water. Conformably with this
rule, at the present day, there are no active volcanoes on this eastern
side of the Cordillera; nor are severe earthquakes experienced here.)
On the opposite and northern side of the valley there is another line
of lava- cliffs at a corresponding height; the valley between being of
considerable breadth, and as nearly as I could estimate 1,500 feet in
depth. This field of lava is confined on both sides by the mountains of
mica-schist, and slopes down rapidly but irregularly to the edge of the
Pampas, where, having a thickness of about two hundred feet, it
terminates against a little range of claystone porphyry. The valley in
this lower part expands into a bay-like, gentle slope, bordered by the
cliffs of lava, which must certainly once have extended across this
wide expanse. The inclination of the streams from Los Arenales to the
mouth of the valley is so great, that at the time (though ignorant of
M. Elie de Beaumont’s researches on the extremely small slope over
which lava can flow, and yet retain a compact structure and
considerable thickness) I concluded that they must subsequently to
their flowing have been upheaved and tilted from the mountains; of this
conclusion I can now entertain not the smallest doubt.
At the mouth of the valley, within the cliffs of the above lava-field,
there are remnants, in the form of separate small hillocks and of lines
of low cliffs, of a considerable deposit of compact white tuff
(quarried for filtering-stones), composed of broken pumice, volcanic
crystals, scales of mica, and fragments of lava. This mass has suffered
much denudation; and the hard mica-schist has been deeply worn, since
the period of its deposition; and this period must have been subsequent
to the denudation of the basaltic lava-streams, as attested by their
encircling cliffs standing at a higher level. At the present day, under
the existing arid climate, ages might roll past without a square yard
of rock of any kind being denuded, except perhaps in the rarely
moistened drainage-channel of the valley. Must we then look back to
that ancient period, when the waves of the sea beat against the eastern
foot of the Cordillera, for a power sufficient to denude extensively,
though superficially, this tufaceous deposit, soft although it be?
There remains only to mention some little water-worn hillocks [BB], a
few hundred feet in height, and mere mole-hills compared with the
gigantic mountains behind them, which rise out of the sloping,
shingle-covered margin of the Pampas. The first little range is
composed of a brecciated purple porphyritic claystone, with obscurely
marked strata dipping at 70 degrees to the S.W.; the other ranges
consist of—a pale-coloured feldspathic porphyry,—a purple claystone
porphyry with grains of quartz,— and a rock almost exclusively composed
of brick-red crystals of feldspar. These outermost small lines of
elevation extend in a N.W. by W. and S.E. by S. direction.
CONCLUDING REMARKS ON THE PORTILLO RANGE.
When on the Pampas and looking southward, and whilst travelling
northward, I could see for very many leagues the red granite and dark
mica-schist forming the crest and eastern flank of the Portillo line.
This great range, according to Dr. Gillies, can be traced with little
interruption for 140 miles southward to the R. Diamante, where it
unites with the western ranges: northward, according to this same
author, it terminates where the R. Mendoza debouches from the
mountains; but a little further north in the eastern part of the Cumbre
section, there are, as we shall hereafter see, some mountain-masses of
a brick-red porphyry, the last injected amidst many other porphyries,
and having so close an analogy with the coarse red granite of the
Portillo line, that I am tempted to believe that they belong to the
same axis of injection; if so, the Portillo line is at least 200 miles
in length. Its height, even in the lowest gap in the road, is 14,365
feet, and some of the pinnacles apparently attain an elevation of about
16,000 feet above the sea. The geological history of this grand chain
appears to me eminently interesting. We may safely conclude, that at a
former period the valley of Tenuyan existed as an arm of the sea, about
twenty-miles in width, bordered on one hand by a ridge or chain of
islets of the black calcareous shales and purple sandstones of the
gypseous formation; and on the other hand, by a ridge or chain of
islets composed of mica-slate, white granite, and perhaps to a partial
extent of red granite. These two chains, whilst thus bordering the old
sea-channel, must have been exposed for a vast lapse of time to
alluvial and littoral action, during which the rocks were shattered,
the fragments rounded, and the strata of conglomerate accumulated to a
thickness of at least fifteen hundred or two thousand feet. The red
orthitic granite now forms, as we have seen, the main part of the
Portillo chain: it is injected in dikes not only into the mica-schist
and white granites, but into the laminated sandstone, which it has
metamorphosed, and which it has thrown off, together with the
conformably overlying coloured beds and stratified conglomerate, at an
angle of forty-five degrees. To have thrown off so vast a pile of
strata at this angle, is a proof that the main part of the red granite
(whether or not portions, as perhaps is probable, previously existed)
was injected in a liquified state after the accumulation both of the
laminated sandstone and of the conglomerate; this conglomerate, we
know, was accumulated, not only after the deposition of the
fossiliferous strata of the Peuquenes line, but after their elevation
and long-continued denudation: and these fossiliferous strata belong to
the early part of the Cretaceous system. Late, therefore, in a
geological sense, as must be the age of the main part of the red
granite, I can conceive nothing more impressive than the eastern view
of this great range, as forcing the mind to grapple with the idea of
the thousands of thousands of years requisite for the denudation of the
strata which originally encased it,—for that the fluidified granite was
once encased, its mineralogical composition and structure, and the bold
conical shape of the mountain-masses, yield sufficient evidence. Of the
encasing strata we see the last vestiges in the coloured beds on the
crest, in the little caps of mica-schist on some of the loftiest
pinnacles, and in the isolated patches of this same rock at
corresponding heights on the now bare and steep flanks.
The lava-streams at the eastern foot of the Portillo are interesting,
not so much from the great denudation which they have suffered at a
comparatively late period as from the evidence they afford by their
inclination taken conjointly with their thickness and compactness, that
after the great range had assumed its present general outline, it
continued to rise as an axis of elevation. The plains extending from
the base of the Cordillera to the Atlantic show that the continent has
been upraised in mass to a height of 3,500 feet, and probably to a much
greater height, for the smooth shingle-covered margin of the Pampas is
prolonged in a gentle unbroken slope far up many of the great valleys.
Nor let it be assumed that the Peuquenes and Portillo ranges have
undergone only movements of elevation; for we shall hereafter see, that
the bottom of the sea subsided several thousand feet during the
deposition of strata, occupying the same relative place in the
Cordillera, with those of the Peuquenes ridge; moreover, we shall see
from the unequivocal evidence of buried upright trees, that at a
somewhat later period, during the formation of the Uspallata chain,
which corresponds geographically with that of the Portillo, there was
another subsidence of many thousand feet: here, indeed, in the valley
of Tenuyan, the accumulation of the coarse stratified conglomerate to a
thickness of fifteen hundred or two thousand feet, offers strong
presumptive evidence of subsidence; for all existing analogies lead to
the belief that large pebbles can be transported only in shallow water,
liable to be affected by currents and movements of undulation—and if
so, the shallow bed of the sea on which the pebbles were first
deposited must necessarily have sunk to allow of the accumulation of
the superincumbent strata. What a history of changes of level, and of
wear and tear, all since the age of the latter secondary formations of
Europe, does the structure of this one great mountain-chain reveal!
PASSAGE OF THE ANDES BY THE CUMBRE OR USPALLATA PASS.
This Pass crosses the Andes about sixty miles north of that just
described: the section given in Plate 1, Section 1/2, is on the same
scale as before, namely, at one-third of an inch to a mile in distance,
and one inch to a mile (or 6,000 feet) in height. Like the last
section, it is a mere sketch, and cannot pretend to accuracy, though
made under favourable circumstances. We will commence as before, with
the western half, of which the main range bears the name of the Cumbre
(that is the Ridge), and corresponds to the Peuquenes line in the
former section; as does the Uspallata range, though on a much smaller
scale, to that of the Portillo. Near the point where the river
Aconcagua debouches on the basin plain of the same name, at a height of
about two thousand three hundred feet above the sea, we meet with the
usual purple and greenish porphyritic claystone conglomerate. Beds of
this nature, alternating with numerous compact and amygdaloidal
porphyries, which have flowed as submarine lavas, and associated with
great mountain- masses of various, injected, non-stratified porphyries,
are prolonged the whole distance up to the Cumbre or central ridge. One
of the commonest stratified porphyries is of a green colour, highly
amygdaloidal with the various minerals described in the preliminary
discussion, and including fine tabular crystals of albite. The
mountain-range north (often with a little westing) and south. The
stratification, wherever I could clearly distinguish it, was inclined
westward or towards the Pacific, and, except near the Cumbre, seldom at
angles above 25 degrees. Only at one spot on this western side, on a
lofty pinnacle not far from the Cumbre, I saw strata apparently
belonging to the gypseous formation, and conformably capping a pile of
stratified porphyries. Hence, both in composition and in
stratification, the structure of the mountains on this western side of
the divortium aquarum, is far more simple than in the corresponding
part of the Peuquenes section. In the porphyritic claystone
conglomerate, the mechanical structure and the planes of stratification
have generally been much obscured and even quite obliterated towards
the base of the series, whilst in the upper parts, near the summits of
the mountains, both are distinctly displayed. In these upper portions
the porphyries are generally lighter coloured. In three places [X, Y,
Z] masses of andesite are exposed: at [Y], this rock contained some
quartz, but the greater part consisted of andesitic porphyry, with only
a few well-developed crystals of albite, and forming a great white
mass, having the external aspect of granite, capped by much dark
unstratified porphyry. In many parts of the mountains, there are dikes
of a green colour, and other white ones, which latter probably spring
from underlying masses of andesite.
The Cumbre, where the road crosses it, is, according to Mr. Pentland,
12,454 feet above the sea; and the neighbouring peaks, composed of dark
purple and whitish porphyries, some obscurely stratified with a
westerly dip, and others without a trace of stratification, must exceed
13,000 feet in height. Descending the eastern slope of the Cumbre, the
structure becomes very complicated, and generally differs on the two
sides of the east and west line of road and section. First we come to a
great mass [A] of nearly vertical, singularly contorted strata,
composed of highly compact red sandstones, and of often calcareous
conglomerates, and penetrated by green, yellow, and reddish dikes; but
I shall presently have an opportunity of describing in some detail an
analogous pile of strata. These vertical beds are abruptly succeeded by
others [B], of apparently nearly the same nature but more
metamorphosed, alternating with porphyries and limestones; these dip
for a short space westward, but there has been here an extraordinary
dislocation, which, on the north side of the road, appears to have
determined the excavation of the north and south valley of the R. de
las Cuevas. On this northern side of the road, the strata [B] are
prolonged till they come in close contact with a jagged lofty mountain
[D] of dark- coloured, unstratified, intrusive porphyry, where the beds
have been more highly inclined and still more metamorphosed. This
mountain of porphyry seems to form a short axis of elevation, for south
of the road in its line there is a hill [C] of porphyritic conglomerate
with absolutely vertical strata.
We now come to the gypseous formation: I will first describe the
structure of the several mountains, and then give in one section a
detailed account of the nature of the rocks. On the north side of the
road, which here runs in an east and west valley, the mountain of
porphyry [D] is succeeded by a hill [E] formed of the upper gypseous
strata tilted, at an angle of between 70 and 80 degrees to the west, by
a uniclinal axis of elevation which does not run parallel to the other
neighbouring ranges, and which is of short length; for on the south
side of the valley its prolongation is marked only by a small flexure
in a pile of strata inclined by a quite separate axis. A little further
on the north and south valley of Horcones enters at right angles our
line of section; its western side is bounded by a hill of gypseous
strata [F] dipping westward at about 45 degrees, and its eastern side
by a mountain of similar strata [G] inclined westward at 70 degrees,
and superimposed by an oblique fault on another mass of the same strata
[H], also inclined westward, but at an angle of about 30 degrees: the
complicated relation of these three masses [F, G, H] is explained by
the structure of a great mountain-range lying some way to the north, in
which a regular anticlinal axis (represented in the section by dotted
lines) is seen, with the strata on its eastern side again bending up
and forming a distinct uniclinal axis, of which the beds marked [H]
form the lower part. This great uniclinal line is intersected, near the
Puente del Inca, by the valley along which the road runs, and the
strata composing it will be immediately described. On the south side of
the road, in the space corresponding with the mountains [E, F, and G],
the strata everywhere dip westward generally at an angle of 30 degrees,
occasionally mounting up to 45 degrees, but not in an unbroken line,
for there are several vertical faults, forming separate uniclinal
masses, all dipping in the same direction,—a form of elevation common
in the Cordillera. We thus see that within a narrow space, the gypseous
strata have been upheaved and crushed together by a great uniclinal,
anticlinal, and one lesser uniclinal line [E] of elevation; and that
between these three lines and the Cumbre, in the sandstones,
conglomerates and porphyritic formation, there have been at least two
or three other great elevatory axes.
The uniclinal axis [I] intersected near the Puente del Inca (of which
the strata at [H] form a part) ranges N. by W. and S. by E., forming a
chain of mountains, apparently little inferior in height to the Cumbre:
the strata, as we have seen, dip at an average angle of 30 degrees to
the west. (At this place, there are some hot and cold springs, the
warmest having a temperature, according to Lieutenant Brand “Travels,”
page 240, of 91 degrees; they emit much gas. According to Mr. Brande,
of the Royal Institution, ten cubical inches contain forty-five grains
of solid matter, consisting chiefly of salt, gypsum, carbonate of lime,
and oxide of iron. The water is charged with carbonic acid and
sulphuretted hydrogen. These springs deposit much tufa in the form of
spherical balls. They burst forth, as do those of Cauquenes, and
probably those of Villa Vicencio, on a line of elevation.) The flanks
of the mountains are here quite bare and steep, affording an excellent
section; so that I was able to inspect the strata to a thickness of
about 4,000 feet, and could clearly distinguish their general nature
for 1,000 feet higher, making a total thickness of 5,000 feet, to which
must be added about 1,000 feet of the inferior strata seen a little
lower down the valley, I will describe this one section in detail,
beginning at the bottom.
1st. The lowest mass is the altered clay-slate described in the
preliminary discussion, and which in this line of section was here
first met with. Lower down the valley, at the R. de las Vacas, I had a
better opportunity of examining it; it is there in some parts well
characterised, having a distinct, nearly vertical, tortuous cleavage,
ranging N.W. and S.E., and intersected by quartz veins: in most parts,
however, it is crystalline and feldspathic, and passes into a true
greenstone often including grains of quartz. The clay-slate, in its
upper half, is frequently brecciated, the embedded angular fragments
being of nearly the same nature with the paste.
2nd. Several strata of purplish porphyritic conglomerate, of no very
great thickness, rest conformably upon the feldspathic slate. A thick
bed of fine, purple, claystone porphyry, obscurely brecciated (but not
of metamorphosed sedimentary origin), and capped by porphyritic
conglomerate, was the lowest bed actually examined in this section at
the Puente del Inca.
3rd. A stratum, eighty feet thick, of hard and very compact impure
whitish limestone, weathering bright red, with included layers
brecciated and re- cemented. Obscure marks of shell are distinguishable
in it.
4th. A red, quartzose, fine-grained conglomerate, with grains of
quartz, and with patches of white earthy feldspar, apparently due to
some process of concretionary crystalline action; this bed is more
compact and metamorphosed than any of the overlying conglomerates.
5th. A whitish cherty limestone, with nodules of bluish argillaceous
limestone.
6th. A white conglomerate, with many particles of quartz, almost
blending into the paste.
7th. Highly siliceous, fine-grained white sandstone.
8th and 9th. Red and white beds not examined.
10th. Yellow, fine-grained, thinly stratified, magnesian (judging from
its slow dissolution in acids) limestone: it includes some white quartz
pebbles, and little cavities, lined with calcareous spar, some
retaining the form of shells.
11th. A bed between twenty and thirty feet thick, quite conformable
with the underlying ones, composed of a hard basis, tinged lilac-grey
porphyritic with NUMEROUS crystals of whitish feldspar, with black mica
and little spots of soft ferruginous matter: evidently a submarine
lava.
12th. Yellow magnesian limestone, as before, part-stained purple.
13th. A most singular rock; basis purplish grey, obscurely crystalline,
easily fusible into a dark green glass, not hard, thickly speckled with
crystals more or less perfect of white carbonate of lime, of red
hydrous oxide of iron, of a white and transparent mineral like
analcime, and of a green opaque mineral like soap-stone; the basis is
moreover amygdaloidal with many spherical balls of white crystallised
carbonate of lime, of which some are coated with the red oxide of iron.
I have no doubt, from the examination of a superincumbent stratum (19),
that this is a submarine lava; though in Northern Chile, some of the
metamorphosed sedimentary beds are almost as crystalline, and of as
varied composition.
14th. Red sandstone, passing in the upper part into a coarse, hard, red
conglomerate, 300 feet thick, having a calcareous cement, and including
grains of quartz and broken crystals of feldspar; basis infusible; the
pebbles consist of dull purplish porphyries, with some of quartz, from
the size of a nut to a man’s head. This is the coarsest conglomerate in
this part of the Cordillera: in the middle there was a white layer not
examined.
15th. Grand thick bed, of a very hard, yellowish-white rock, with a
crystalline feldspathic base, including large crystals of white
feldspar, many little cavities mostly full of soft ferruginous matter,
and numerous hexagonal plates of black mica. The upper part of this
great bed is slightly cellular; the lower part compact: the thickness
varied a little in different parts. Manifestly a submarine lava; and is
allied to bed 11.
16th and 17th. Dull purplish, calcareous, fine-grained, compact
sandstones, which pass into coarse white conglomerates with numerous
particles of quartz.
18th. Several alternations of red conglomerate, purplish sandstone, and
submarine lava, like that singular rock forming bed 13.
19th. A very heavy, compact, greenish-black stone, with a fine-grained
obviously crystalline basis, containing a few specks of white
calcareous spar, many specks of the crystallised hydrous red oxide of
iron, and some specks of a green mineral; there are veins and nests
filled with epidote: certainly a submarine lava.
20th. Many thin strata of compact, fine-grained, pale purple sandstone.
21st. Gypsum in a nearly pure state, about three hundred feet in
thickness: this bed, in its concretions of anhydrite and layers of
small blackish crystals of carbonate of lime, exactly resembles the
great gypseous beds in the Peuquenes range.
22nd. Pale purple and reddish sandstone, as in bed 20: about three
hundred feet in thickness.
23rd. A thick mass composed of layers, often as thin as paper and
convoluted, of pure gypsum with others very impure, of a purplish
colour.
24th. Pure gypsum, thick mass.
25th. Red sandstones, of great thickness.
26th. Pure gypsum, of great thickness.
27th. Alternating layers of pure and impure gypsum, of great thickness.
I was not able to ascend to these few last great strata, which compose
the neighbouring loftiest pinnacles. The thickness, from the lowest to
the uppermost bed of gypsum, cannot be less than 2,000 feet: the beds
beneath I estimated at 3,000 feet, and this does not include either the
lower parts of the porphyritic conglomerate, or the altered clay-slate;
I conceive the total thickness must be about six thousand feet. I
distinctly observed that not only the gypsum, but the alternating
sandstones and conglomerates were lens-shaped, and repeatedly thinned
out and replaced each other: thus in the distance of about a mile, a
bed 300 feet thick of sandstone between two beds of gypsum, thinned out
to nothing and disappeared. The lower part of this section differs
remarkably,—in the much greater diversity of its mineralogical
composition,—in the abundance of calcareous matter,—in the greater
coarseness of some of the conglomerates,—and in the numerous particles
and well-rounded pebbles, sometimes of large size, of quartz,— from any
other section hitherto described in Chile. From these peculiarities and
from the lens-form of the strata, it is probable that this great pile
of strata was accumulated on a shallow and very uneven bottom, near
some pre-existing land formed of various porphyries and quartz-rock.
The formation of porphyritic claystone conglomerate does not in this
section attain nearly its ordinary thickness; this may be PARTLY
attributed to the metamorphic action having been here much less
energetic than usual, though the lower beds have been affected to a
certain degree. If it had been as energetic as in most other parts of
Chile, many of the beds of sandstone and conglomerate, containing
rounded masses of porphyry, would doubtless have been converted into
porphyritic conglomerate; and these would have alternated with, and
even blended into, crystalline and porphyritic strata without a trace
of mechanical structure,—namely, into those which, in the present state
of the section, we see are unquestionably submarine lavas.
The beds of gypsum, together with the red alternating sandstones and
conglomerates, present so perfect and curious a resemblance with those
seen in our former section in the basin-valley of Yeso, that I cannot
doubt the identity of the two formations: I may add, that a little
westward of the P. del Inca, a mass of gypsum passed into a
fine-grained, hard, brown sandstone, which contained some layers of
black, calcareous, compact, shaly rock, precisely like that seen in
such vast masses on the Peuquenes range.
Near the Puente del Inca, numerous fragments of limestone, containing
some fossil remains, were scattered on the ground: these fragments so
perfectly resemble the limestone of bed No. 3, in which I saw
impressions of shells, that I have no doubt they have fallen from it.
The yellow magnesian limestone of bed No. 10, which also includes
traces of shells, has a different appearance. These fossils (as named
by M. d’Orbigny) consist of:—
Gryphaea, near to G. Couloni (Neocomian formation). Arca, perhaps A.
Gabrielis, d’Orbigny, “Pal. Franc.” (Neocomian formation).
Mr. Pentland made a collection of shells from this same spot, and Von
Buch considers them as consisting of:—
Trigonia, resembling in form T. costata. Pholadomya, like one found by
M. Dufresnoy near Alencon. Isocardi excentrica, Voltz., identical with
that from the Jura. (“Description Phys. des Iles Can.” page 472.)
Two of these shells, namely, the Gryphaea and Trigonia, appear to be
identical with species collected by Meyen and myself on the Peuquenes
range; and in the opinion of Von Buch and M. d’Orbigny, the two
formations belong to the same age. I must here add, that Professor E.
Forbes, who has examined my specimens from this place and from the
Peuquenes range, has likewise a strong impression that they indicate
the Cretaceous period, and probably an early epoch in it: so that all
the palaeontologists who have seen these fossils nearly coincide in
opinion regarding their age. The limestone, however, with these fossils
here lies at the very base of the formation, just above the porphyritic
conglomerate, and certainly several thousand feet lower in the series,
than the equivalent, fossiliferous, black, shaly rocks high up on the
Peuquenes range.
It is well worthy of remark that these shells, or at least those of
which I saw impressions in the limestone (bed No. 3), must have been
covered up, on the LEAST computation, by 4,000 feet of strata: now we
know from Professor E. Forbes’s researches, that the sea at greater
depths than 600 feet becomes exceedingly barren of organic beings,—a
result quite in accordance with what little I have seen of deep-sea
soundings. Hence, after this limestone with its shells was deposited,
the bottom of the sea where the main line of the Cordillera now stands,
must have subsided some thousand feet to allow of the deposition of the
superincumbent submarine strata. Without supposing a movement of this
kind, it would, moreover, be impossible to understand the accumulation
of the several lower strata of COARSE, well-rounded conglomerates,
which it is scarcely possible to believe were spread out in profoundly
deep water, and which, especially those containing pebbles of quartz,
could hardly have been rounded in submarine craters and afterwards
ejected from them, as I believe to have been the case with much of the
porphyritic conglomerate formation. I may add that, in Professor
Forbes’s opinion, the above-enumerated species of mollusca probably did
not live at a much greater depth than twenty fathoms, that is only 120
feet.
To return to our section down the valley; standing on the great N. by
W. and S. by E. uniclinal axis of the Puente del Inca, of which a
section has just been given, and looking north-east, greater tabular
masses of gypseous formation (KK) could be seen in the distance, very
slightly inclined towards the east. Lower down the valley, the
mountains are almost exclusively composed of porphyries, many of them
of intrusive origin and non-stratified, others stratified, but with the
stratification seldom distinguishable except in the upper parts.
Disregarding local disturbances, the beds are either horizontal or
inclined at a small angle eastwards: hence, when standing on the plain
of Uspallata and looking to the west or backwards, the Cordillera
appear composed of huge, square, nearly horizontal, tabular masses: so
wide a space, with such lofty mountains so equably elevated, is rarely
met with within the Cordillera. In this line of section, the interval
between the Puente del Inca and the neighbourhood of the Cumbre,
includes all the chief axes of dislocation.
The altered clay-slate formation, already described, is seen in several
parts of the valley as far down as Las Vacas, underlying the
porphyritic conglomerate. At the Casa de Pujios [L], there is a hummock
of (andesitic?) granite; and the stratification of the surrounding
mountains here changes from W. by S. to S.W. Again, near the R. Vacas
there is a larger formation of (andesitic?) granite [M], which sends a
meshwork of veins into the superincumbent clay-slate, and which locally
throws off the strata, on one side to N.W. and on the other to S.E. but
not at a high angle: at the junction, the clay-slate is altered into
fine-grained greenstone. This granitic axis is intersected by a green
dike, which I mention, because I do not remember having elsewhere seen
dikes in this lowest and latest intrusive rock. From the R. Vacas to
the plain of Uspallata, the valley runs N.E., so that I have had to
contract my section; it runs exclusively through porphyritic rocks. As
far as the Pass of Jaula, the claystone conglomerate formation, in most
parts highly porphyritic, and crossed by numerous dikes of greenstone
porphyry, attains a great thickness: there is also much intrusive
porphyry. From the Jaula to the plain, the stratification has been in
most places obliterated, except near the tops of some of the mountains;
and the metamorphic action has been extremely great. In this space, the
number and bulk of the intrusive masses of differently coloured
porphyries, injected one into another and intersected by dikes, is
truly extraordinary. I saw one mountain of whitish porphyry, from which
two huge dikes, thinning out, branched DOWNWARDS into an adjoining
blackish porphyry. Another hill of white porphyry, which had burst
through dark- coloured strata, was itself injected by a purple,
brecciated, and recemented porphyry, both being crossed by a green
dike, and both having been upheaved and injected by a granitic dome.
One brick-red porphyry, which above the Jaula forms an isolated mass in
the midst of the porphyritic conglomerate formation, and lower down the
valley a magnificent group of peaked mountains, differs remarkably from
all the other porphyries. It consists of a red feldspathic base,
including some rather large crystals of red feldspar, numerous large
angular grains of quartz, and little bits of a soft green mineral
answering in most of its characters to soapstone. The crystals of red
feldspar resemble in external appearance those of orthite, though, from
being partially decomposed, I was unable to measure them; and they
certainly are quite unlike the variety, so abundantly met with in
almost all the other rocks of this line of section, and which, wherever
I tried it, cleaved like albite. This brick-red porphyry appears to
have burst through all the other porphyries, and numerous red dikes
traversing the neighbouring mountains have proceeded from it: in some
few places, however, it was intersected by white dikes. From this
posteriority of intrusive origin,—from the close general resemblance
between this red porphyry and the red granite of the Portillo line, the
only difference being that the feldspar here is less perfectly
granular, and that soapstone replaces the mica, which is there
imperfect and passes into chlorite,—and from the Portillo line a little
southward of this point appearing to blend (according to Dr. Gillies)
into the western ranges,—I am strongly urged to believe (as formerly
remarked) that the grand mountain-masses composed of this brick-red
porphyry belong to the same axis of injection with the granite of the
Portillo line; if so, the injection of this porphyry probably took
place, as long subsequently to the several axes of elevation in the
gypseous formation near the Cumbre, as the injection of the Portillo
granite has been shown to have been subsequent to the elevation of the
gypseous strata composing the Peuquenes range; and this interval, we
have seen, must have been a very long one.
The Plain of Uspallata has been briefly described in Chapter 3; it
resembles the basin-plains of Chile; it is ten or fifteen miles wide,
and is said to extend for 180 miles northward; its surface is nearly
six thousand feet above the sea; it is composed, to a thickness of some
hundred feet of loosely aggregated, stratified shingle, which is
prolonged with a gently sloping surface up the valleys in the mountains
on both sides. One section in this plain [Z] is interesting, from the
unusual circumstance of alternating layers of almost loose red and
white sand with lines of pebbles (from the size of a nut to that of an
apple), and beds of gravel, being inclined at an angle of 45 degrees,
and in some spots even at a higher angle. (I find that Mr. Smith of
Jordan Hill has described (“Edinburgh New Philosophical Journal” volume
25 page 392) beds of sand and gravel, near Edinburgh, tilted at an
angle of 60 degrees, and dislocated by miniature faults.) These beds
are dislocated by small faults: and are capped by a thick mass of
horizontally stratified gravel, evidently of subaqueous origin. Having
been accustomed to observe the irregularities of beds accumulated under
currents, I feel sure that the inclination here has not been thus
produced. The pebbles consist chiefly of the brick-red porphyry just
described and of white granite, both probably derived from the ranges
to the west, and of altered clay-slate and of certain porphyries,
apparently belonging to the rocks of the Uspallata chain. This plain
corresponds geographically with the valley of Tenuyan between the
Portillo and Peuquenes ranges; but in that valley the shingle, which
likewise has been derived both from the eastern and western ranges, has
been cemented into a hard conglomerate, and has been throughout tilted
at a considerable inclination; the gravel there apparently attains a
much greater thickness, and is probably of higher antiquity.
THE USPALLATA RANGE.
The road by the Villa Vicencio Pass does not strike directly across the
range, but runs for some leagues northward along its western base: and
I must briefly describe the rocks here seen, before continuing with the
coloured east and west section. At the mouth of the valley of Canota,
and at several points northwards, there is an extensive formation of a
glossy and harsh, and of a feldspathic clay-slate, including strata of
grauwacke, and having a tortuous, nearly vertical cleavage, traversed
by numerous metalliferous veins and others of quartz. The clay-slate is
in many parts capped by a thick mass of fragments of the same rock,
firmly recemented; and both together have been injected and broken up
by very numerous hillocks, ranging north and south, of lilac, white,
dark and salmon- coloured porphyries: one steep, now denuded, hillock
of porphyry had its face as distinctly impressed with the angles of a
fragmentary mass of the slate, with some of the points still remaining
embedded, as sealing-wax could be by a seal. At the mouth of this same
valley of Canota, in a fine escarpment having the strata dipping from
50 to 60 degrees to the N.E. (Nearly opposite to this escarpment, there
is another corresponding one, with the strata dipping not to the
exactly opposite point, or S.W., but to S.S.W.: consequently the two
escarpments trend towards each other, and some miles southward they
become actually united: this is a form of elevation which I have not
elsewhere seen.), the clay-slate formation is seen to be covered
by—(1st) a purple, claystone porphyry resting unconformably in some
parts on the solid slate, and in others on a thick fragmentary mass;
(2nd), a conformable stratum of compact blackish rock, having a
spheroidal structure, full of minute acicular crystals of glassy
feldspar, with red spots of oxide of iron; (3rd), a great stratum of
purplish-red claystone porphyry, abounding with crystals of opaque
feldspar, and laminated with thin, parallel, often short, layers, and
likewise with great irregular patches of white, earthy,
semi-crystalline feldspar; this rock (which I noticed in other
neighbouring places) perfectly resembles a curious variety described at
Port Desire, and occasionally occurs in the great porphyritic
conglomerate formation of Chile; (4th), a thin stratum of greenish
white, indurated tuff, fusible and containing broken crystals and
particles of porphyries; (5th), a grand mass, imperfectly columnar and
divided into three parallel and closely joined strata, of
cream-coloured claystone porphyry; (6th), a thick stratum of
lilac-coloured porphyry, which I could see was capped by another bed of
the cream-coloured variety; I was unable to examine the still higher
parts of the escarpment. These conformably stratified porphyries,
though none are either vesicular are amygdaloidal, have evidently
flowed as submarine lavas: some of them are separated from each other
by seams of indurated tuff, which, however, are quite insignificant in
thickness compared with the porphyries. This whole pile resembles, but
not very closely, some of the less brecciated parts of the great
porphyritic conglomerate formation of Chile; but it does not probably
belong to the same age, as the porphyries here rest unconformably on
the altered feldspathic clay-slate, whereas the porphyritic
conglomerate formation alternates with and rests conformably on it.
These porphyries, moreover, with the exception of the one blackish
stratum, and of the one indurated, white tufaceous bed, differ from the
beds composing the Uspallata range in the line of the Villa Vicencio
Pass.
I will now give, first, a sketch of the structure of the range, as
represented in the section, and will then describe its composition and
interesting history. At its western foot, a hillock [N] is seen to rise
out of the plain, with its strata dipping at 70 degrees to the west,
fronted by strata [O] inclined at 45 degrees to the east, thus forming
a little north and south anticlinal axis. Some other little hillocks of
similar composition, with their strata highly inclined, range N.E. and
S.W., obliquely to the main Uspallata line. The cause of these
dislocations, which, though on a small scale, have been violent and
complicated, is seen to lie in hummocks of lilac, purple and red
porphyries, which have been injected in a liquified state through and
into the underlying clay-slate formation. Several dykes were exposed
here, but in no other part, that I saw of this range. As the strata
consist of black, white, greenish and brown-coloured rocks, and as the
intrusive porphyries are so brightly tinted, a most extraordinary view
was presented, like a coloured geological drawing. On the gently
inclined main western slope [PP], above the little anticlinal ridges
just mentioned, the strata dip at an average angle of 25 degrees to the
west; the inclination in some places being only 19 degrees, in some few
others as much as 45 degrees. The masses having these different
inclinations, are separated from each other by parallel vertical faults
[as represented at Pa], often giving rise to separate, parallel,
uniclinal ridges. The summit of the main range is broad and undulatory,
with the stratification undulatory and irregular: in a few places
granitic and porphyritic masses [Q] protrude, which, from the small
effect they have locally produced in deranging the strata, probably
form the upper points of a regular, great underlying dome. These
denuded granitic points, I estimated at about nine thousand feet in
height above the sea. On the eastern slope, the strata in the upper
part are regularly inclined at about 25 degrees to the east, so that
the summit of this chain, neglecting small irregularities, forms a
broad anticlinal axis. Lower down, however, near Los Hornillos [R],
there is a well-marked synclinal axis, beyond which the strata are
inclined at nearly the same angle, namely from 20 to 30 degrees,
inwards or westward. Owing to the amount of denudation which this chain
has suffered, the outline of the gently inclined eastern flank scarcely
offers the slightest indication of this synclinal axis. The stratified
beds, which we have hitherto followed across the range, a little
further down are seen to lie, I believe unconformably, on a broad
mountainous band of clay-slate and grauwacke. The strata and laminae of
this latter formation, on the extreme eastern flank, are generally
nearly vertical; further inwards they become inclined from 45 to 80
degrees to the west: near Villa Vicencio [S] there is apparently an
anticlinal axis, but the structure of this outer part of the clay-slate
formation is so obscure, that I have not marked the planes of
stratification in the section. On the margin of the Pampas, some low,
much dislocated spurs of this same formation, project in a north-
easterly line, in the same oblique manner as do the ridges on the
western foot, and as is so frequently the case with those at the base
of the main Cordillera.
I will now describe the nature of the beds, beginning at the base on
the eastern side. First, for the clay-slate formation: the slate is
generally hard and bluish, with the laminae coated by minute micaceous
scales; it alternates many times with a coarse-grained, greenish
grauwacke, containing rounded fragments of quartz and bits of slate in
a slightly calcareous basis. The slate in the upper part generally
becomes purplish, and the cleavage so irregular that the whole consists
of mere splinters. Transverse veins of quartz are numerous. At the
Calera, some leagues distant, there is a dark crystalline limestone,
apparently included in this formation. With the exception of the
grauwacke being here more abundant, and the clay-slate less altered,
this formation closely resembles that unconformably underlying the
porphyries at the western foot of this same range; and likewise that
alternating with the porphyritic conglomerate in the main Cordillera.
This formation is a considerable one, and extends several leagues
southward to near Mendoza: the mountains composed of it rise to a
height of about two thousand feet above the edge of the Pampas, or
about seven thousand feet above the sea. (I infer this from the height
of V. Vicencio, which was ascertained by Mr. Miers to be 5,328 feet
above the sea.)
Secondly: the most usual bed on the clay-slate is a coarse, white,
slightly calcareous conglomerate, of no great thickness, including
broken crystals of feldspar, grains of quartz, and numerous pebbles of
brecciated claystone porphyry, but without any pebbles of the
underlying clay-slate. I nowhere saw the actual junction between this
bed and the clay-slate, though I spent a whole day in endeavouring to
discover their relations. In some places I distinctly saw the white
conglomerate and overlying beds inclined at from 25 to 30 degrees to
the west, and at the bottom of the same mountain, the clay-slate and
grauwacke inclined to the same point, but at an angle from 70 to 80
degrees: in one instance, the clay-slate dipped not only at a different
angle, but to a different point from the overlying formation. In these
cases the two formations certainly appeared quite unconformable:
moreover, I found in the clay-slate one great, vertical, dike-like
fissure, filled up with an indurated whitish tuff, quite similar to
some of the upper beds presently to be described; and this shows that
the clay-slate must have been consolidated and dislocated before their
deposition. On the other hand, the stratification of the slate and
grauwacke, in some cases gradually and entirely disappeared in
approaching the overlying white conglomerate; in other cases the
stratification of the two formations became strictly conformable; and
again in other cases, there was some tolerably well characterised
clay-slate lying above the conglomerate. (The coarse, mechanical
structure of many grauwackes has always appeared to me a difficulty;
for the texture of the associated clay-slate and the nature of the
embedded organic remains where present, indicate that the whole has
been a deep-water deposit. Whence have the sometimes included angular
fragments of clay-slate, and the rounded masses of quartz and other
rocks, been derived? Many deep-water limestones, it is well known, have
been brecciated, and then firmly recemented.) The most probable
conclusion appears to be, that after the clay-slate formation had been
dislocated and tilted, but whilst under the sea, a fresh and more
recent deposition of clay-slate took place, on which the white
conglomerate was conformably deposited, with here and there a thin
intercalated bed of clay-slate. On this view the white conglomerates
and the presently to be described tuffs and lavas are really
unconformable to the main part of the clay-slate; and this, as we have
seen, certainly is the case with the clay-stone lavas in the valley of
Canota, at the western and opposite base of the range.
Thirdly: on the white conglomerate, strata several hundred feet in
thickness are superimposed, varying much in nature in short distances:
the commonest variety is a white, much indurated tuff, sometimes
slightly calcareous, with ferruginous spots and water-lines, often
passing into whitish or purplish compact, fine-grained grit or
sandstones; other varieties become semi-porcellanic, and tinted faint
green or blue; others pass into an indurated shale: most of these
varieties are easily fusible.
Fourthly: a bed, about one hundred feet thick of a compact, partially
columnar, pale-grey, feldspathic lava, stained with iron, including
very numerous crystals of opaque feldspar, and with some crystallised
and disseminated calcareous matter. The tufaceous stratum on which this
feldspathic lava rests is much hardened, stained purple, and has a
spherico-concretionary structure; it here contains a good many pebbles
of claystone porphyry.
Fifthly: thin beds, 400 feet in thickness, varying much in nature,
consisting of white and ferruginous tuffs, in some parts having a
concretionary structure, in others containing rounded grains and a few
pebbles of quartz; also passing into hard gritstones and into greenish
mudstones: there is, also, much of a bluish-grey and green
semi-porcellanic stone.
Sixthly: a volcanic stratum, 250 feet in thickness, of so varying a
nature that I do not believe a score of specimens would show all the
varieties; much is highly amygdaloidal, much compact; there are
greenish, blackish, purplish, and grey varieties, rarely including
crystals of green augite and minute acicular ones of feldspar, but
often crystals and amygdaloidal masses of white, red, and black
carbonate of lime. Some of the blackish varieties of this rock have a
conchoidal fracture and resemble basalt; others have an irregular
fracture. Some of the grey and purplish varieties are thickly speckled
with green earth and with white crystalline carbonate of lime; others
are largely amygdaloidal with green earth and calcareous spar. Again,
other earthy varieties, of greenish, purplish and grey tints, contain
much iron, and are almost half composed of amygdaloidal balls of dark
brown bole, of a whitish indurated feldspathic matter, of bright green
earth, of agate, and of black and white crystallised carbonate of lime.
All these varieties are easily fusible. Viewed from a distance, the
line of junction with the underlying semi-porcellanic strata was
distinct; but when examined closely, it was impossible to point out
within a foot where the lava ended and where the sedimentary mass
began: the rock at the time of junction was in most places hard, of a
bright green colour, and abounded with irregular amygdaloidal masses of
ferruginous and pure calcareous spar, and of agate.
Seventhly: strata, eighty feet in thickness, of various indurated
tuffs, as before; many of the varieties have a fine basis including
rather coarse extraneous particles; some of them are compact and
semi-porcellanic, and include vegetable impressions.
Eighthly: a bed, about fifty feet thick, of greenish-grey, compact,
feldspathic lava, with numerous small crystals of opaque feldspar,
black augite, and oxide of iron. The junction with the bed on which it
rested, was ill defined; balls and masses of the feldspathic rock being
enclosed in much altered tuff.
Ninthly: indurated tuffs, as before.
Tenthly: a conformable layer, less than two feet in thickness, of
pitchstone, generally brecciated, and traversed by veins of agate and
of carbonate of lime: parts are composed of apparently concretionary
fragments of a more perfect variety, arranged in horizontal lines in a
less perfectly characterised variety. I have much difficulty in
believing that this thin layer of pitchstone flowed as lava.
Eleventhly: sedimentary and tufaceous beds as before, passing into
sandstone, including some conglomerate: the pebbles in the latter are
of claystone porphyry, well rounded, and some as large as
cricket-balls.
Twelfthly: a bed of compact, sonorous, feldspathic lava, like that of
bed No. 8, divided by numerous joints into large angular blocks.
Thirteenthly: sedimentary beds as before.
Fourteenthly: a thick bed of greenish or greyish black, compact basalt
(fusing into a black enamel), with small crystals, occasionally
distinguishable, of feldspar and augite: the junction with the
underlying sedimentary bed, differently from that in most of the
foregoing streams, here was quite distinct:—the lava and tufaceous
matter preserving their perfect characters within two inches of each
other. This rock closely resembles certain parts of that varied and
singular lava-stream No. 6; it likewise resembles, as we shall
immediately see, many of the great upper beds on the western flank and
on the summit of this range.
The pile of strata here described attains a great thickness; and above
the last-mentioned volcanic stratum, there were several other great
tufaceous beds alternating with submarine lavas, which I had not time
to examine; but a corresponding series, several thousand feet in
thickness, is well exhibited on the crest and western flank of the
range. Most of the lava- streams on the western side are of a jet-black
colour and basaltic nature; they are either compact and fine-grained,
including minute crystals of augite and feldspar, or they are
coarse-grained and abound with rather large coppery-brown crystals of
an augitic mineral. (Very easily fusible into a jet-black bead,
attracted by the magnet: the crystals are too much tarnished to be
measured by the goniometer.) Another variety was of a dull- red colour,
having a claystone brecciated basis, including specks of oxide of iron
and of calcareous spar, and amygdaloidal with green earth: there were
apparently several other varieties. These submarine lavas often exhibit
a spheroidal, and sometimes an imperfect columnar structure: their
upper junctions are much more clearly defined than their lower
junctions; but the latter are not so much blended into the underlying
sedimentary beds as is the case in the eastern flank. On the crest and
western flank of the range, the streams, viewed as a whole, are mostly
basaltic; whilst those on the eastern side, which stand lower in the
series, are, as we have seen, mostly feldspathic.
The sedimentary strata alternating with the lavas on the crest and
western side, are of an almost infinitely varying nature; but a large
proportion of them closely resemble those already described on the
eastern flank: there are white and brown, indurated, easily fusible
tuffs,—some passing into pale blue and green semi-porcellanic
rocks,—others into brownish and purplish sandstones and gritstones,
often including grains of quartz,— others into mudstone containing
broken crystals and particles of rock, and occasionally single large
pebbles. There was one stratum of a bright red, coarse, volcanic
gritstone; another of conglomerate; another of a black, indurated,
carbonaceous shale marked with imperfect vegetable impressions; this
latter bed, which was thin, rested on a submarine lava, and followed
all the considerable inequalities of its upper surface. Mr. Miers
states that coal has been found in this range. Lastly, there was a bed
(like No. 10 on the eastern flank) evidently of sedimentary origin, and
remarkable from closely approaching in character to an imperfect
pitchstone, and from including extremely thin layers of perfect
pitchstone, as well as nodules and irregular fragments (but not
resembling extraneous fragments) of this same rock arranged in
horizontal lines: I conceive that this bed, which is only a few feet in
thickness, must have assumed its present state through metamorphic and
concretionary action. Most of these sedimentary strata are much
indurated, and no doubt have been partially metamorphosed: many of them
are extraordinarily heavy and compact; others have agate and
crystalline carbonate of lime disseminated throughout them. Some of the
beds exhibit a singular concretionary arrangement, with the curves
determined by the lines of fissure. There are many veins of agate and
calcareous spar, and innumerable ones of iron and other metals, which
have blackened and curiously affected the strata to considerable
distances on both sides.
Many of these tufaceous beds resemble, with the exception of being more
indurated, the upper beds of the Great Patagonian tertiary formation,
especially those variously coloured layers high up the River Santa
Cruz, and in a remarkable degree the tufaceous formation at the
northern end of Chiloe. I was so much struck with this resemblance,
that I particularly looked out for silicified wood, and found it under
the following extraordinary circumstances. High up on this western
flank, at a height estimated at 7,000 feet above the sea, in a broken
escarpment of thin strata, composed of compact green gritstone passing
into a fine mudstone, and alternating with layers of coarser, brownish,
very heavy mudstone, including broken crystals and particles of rock
almost blended together, I counted the stumps of fifty-two trees. (For
the information of any future traveller, I will describe the spot in
detail. Proceeding eastward from the Agua del Zorro, and afterwards
leaving on the north side of the road a rancho attached to some old
goldmines, you pass through a gully with low but steep rocks on each
hand: the road then bends, and the ascent becomes steeper. A few
hundred yards farther on, a stone’s throw on the south side of the
road, the white calcareous stumps may be seen. The spot is about half a
mile east of the Agua del Zorro.) They projected between two and five
feet above the ground, and stood at exactly right angles to the strata,
which were here inclined at an angle of about 25 degrees to the west.
Eleven of these trees were silicified and well preserved; Mr. R. Brown
has been so kind as to examine the wood when sliced and polished; he
says it is coniferous, partaking of the characters of the Araucarian
tribe, with some curious points of affinity with the Yew. The bark
round the trunks must have been circularly furrowed with irregular
lines, for the mudstone round them is thus plainly marked. One cast
consisted of dark argillaceous limestone; and forty of them of coarsely
crystallised carbonate of lime, with cavities lined by quartz crystals:
these latter white calcareous columns do not retain any internal
structure, but their external form plainly shows their origin. All the
stumps have nearly the same diameter, varying from one foot to eighteen
inches; some of them stand within a yard of each other; they are
grouped in a clump within a space of about sixty yards across, with a
few scattered round at the distance of 150 yards. They all stand at
about the same level. The longest stump stood seven feet out of the
ground: the roots, if they are still preserved, are buried and
concealed. No one layer of the mudstone appeared much darker than the
others, as if it had formerly existed as soil, nor could this be
expected, for the same agents which replaced with silex and lime the
wood of the trees, would naturally have removed all vegetable matter
from the soil. Besides the fifty-two upright trees, there were a few
fragments, like broken branches, horizontally embedded. The surrounding
strata are crossed by veins of carbonate of lime, agate, and oxide of
iron; and a poor gold vein has been worked not far from the trees.
The green and brown mudstone beds including the trees, are conformably
covered by much indurated, compact, white or ferruginous tuffs, which
pass upwards into a fine-grained, purplish sedimentary rock: these
strata, which, together, are from four to five hundred feet in
thickness, rest on a thick bed of submarine lava, and are conformably
covered by another great mass of fine-grained basalt, which I estimated
at 1,000 feet in thickness, and which probably has been formed by more
than one stream. (This rock is quite black, and fuses into a black
bead, attracted strongly by the magnet; it breaks with a conchoidal
fracture; the included crystals of augite are distinguishable by the
naked eye, but are not perfect enough to be measured: there are many
minute acicular crystals of glassy feldspar.) Above this mass I could
clearly distinguish five conformable alternations, each several hundred
feet in thickness, of stratified sedimentary rocks and lavas, such as
have been previously described. Certainly the upright trees have been
buried under several thousand feet in thickness of matter, accumulated
under the sea. As the trees obviously must once have grown on dry land,
what an enormous amount of subsidence is thus indicated! Nevertheless,
had it not been for the trees there was no appearance which would have
led any one even to have conjectured that these strata had subsided. As
the land, moreover, on which the trees grew, is formed of subaqueous
deposits, of nearly if not quite equal thickness with the
superincumbent strata, and as these deposits are regularly stratified
and fine-grained, not like the matter thrown up on a sea-beach, a
previous upward movement, aided no doubt by the great accumulation of
lavas and sediment, is also indicated. (At first I imagined, that the
strata with the trees might have been accumulated in a lake: but this
seems highly improbable; for, first, a very deep lake was necessary to
receive the matter below the trees, then it must have been drained for
their growth, and afterwards re-formed and made profoundly deep, so as
to receive a subsequent accumulation of matter SEVERAL THOUSAND feet in
thickness. And all this must have taken place necessarily before the
formation of the Uspallata range, and therefore on the margin of the
wide level expanse of the Pampas! Hence I conclude, that it is
infinitely more probable that the strata were accumulated under the
sea: the vast amount of denudation, moreover, which this range has
suffered, as shown by the wide valleys, by the exposure of the very
trees and by other appearances, could have been effected, I conceive,
only by the long-continued action of the sea; and this shows that the
range was either upheaved from under the sea, or subsequently let down
into it. From the natural manner in which the stumps (fifty-two in
number) are GROUPED IN A CLUMP, and from their all standing vertically
to the strata, it is superfluous to speculate on the chance of the
trees having been drifted from adjoining land, and deposited upright: I
may, however, mention that the late Dr. Malcolmson assured me, that he
once met in the Indian Ocean, fifty miles from land, several cocoa-nut
trees floating upright, owing to their roots being loaded with earth.)
In nearly the middle of the range, there are some hills [Q], before
alluded to, formed of a kind of granite externally resembling andesite,
and consisting of a white, imperfectly granular, feldspathic basis,
including some perfect crystals apparently of albite (but I was unable
to measure them), much black mica, epidote in veins, and very little or
no quartz. Numerous small veins branch from this rock into the
surrounding strata; and it is a singular fact that these veins, though
composed of the same kind of feldspar and small scales of mica as in
the solid rock, abound with innumerable minute ROUNDED grains of
quartz: in the veins or dikes also, branching from the great granitic
axis in the peninsula of Tres Montes, I observed that quartz was more
abundant in them than in the main rock: I have heard of other analogous
cases: can we account for this fact, by the long-continued vicinity of
quartz when cooling, and by its having been thus more easily sucked
into fissures than the other constituent minerals of granite? (See a
paper by M. Elie de Beaumont, “Soc. Philomath.” May 1839 “L’Institut.”
1839 page 161.) The strata encasing the flanks of these granitic or
andesite masses, and forming a thick cap on one of their summits,
appear originally to have been of the same tufaceous nature with the
beds already described, but they are now changed into porcellanic,
jaspery, and crystalline rocks, and into others of a white colour with
a harsh texture, and having a siliceous aspect, though really of a
feldspathic nature and fusible. Both the granitic intrusive masses and
the encasing strata are penetrated by innumerable metallic veins,
mostly ferruginous and auriferous, but some containing copper-pyrites
and a few silver: near the veins, the rocks are blackened as if blasted
by gunpowder. The strata are only slightly dislocated close round these
hills, and hence, perhaps, it may be inferred that the granitic masses
form only the projecting points of a broad continuous axis-dome, which
has given to the upper parts of this range its anticlinal structure.
CONCLUDING REMARKS ON THE USPALLATA RANGE.
I will not attempt to estimate the total thickness of the pile of
strata forming this range, but it must amount to many thousand feet.
The sedimentary and tufaceous beds have throughout a general
similarity, though with infinite variations. The submarine lavas in the
lower part of the series are mostly feldspathic, whilst in the upper
part, on the summit and western flank, they are mostly basaltic. We are
thus reminded of the relative position in most recent volcanic
districts of the trachytic and basaltic lavas,—the latter from their
greater weight having sunk to a lower level in the earth’s crust, and
having consequently been erupted at a later period over the lighter and
upper lavas of the trachytic series. (See on this subject, “Volcanic
Islands” etc. by the Author.) Both the basaltic and feldspathic
submarine streams are very compact; none being vesicular, and only a
few amygdaloidal: the effects which some of them, especially those low
in the series, have produced on the tufaceous beds over which they have
flowed is highly curious. Independently of this local metamorphic
action, all the strata undoubtedly display an indurated and altered
character; and all the rocks of this range—the lavas, the alternating
sediments, the intrusive granite and porphyries, and the underlying
clay- slate—are intersected by metalliferous veins. The lava-strata can
often be seen extending for great distances, conformably with the under
and overlying beds; and it was obvious that they thickened towards the
west. Hence the points of eruption must have been situated westward of
the present range, in the direction of the main Cordillera: as,
however, the flanks of the Cordillera are entirely composed of various
porphyries, chiefly claystone and greenstone, some intrusive, and
others belonging to the porphyritic conglomerate formation, but all
quite unlike these submarine lava-streams, we must in all probability
look to the plain of Uspallata for the now deeply buried points of
eruption.
Comparing our section of the Uspallata range with that of the Cumbre,
we see, with the exception of the underlying clay-slate, and perhaps of
the intrusive rocks of the axes, a striking dissimilarity in the strata
composing them. The great porphyritic conglomerate formation has not
extended as far as this range; nor have we here any of the gypseous
strata, the magnesian and other limestones, the red sandstones, the
siliceous beds with pebbles of quartz, and comparatively little of the
conglomerates, all of which form such vast masses over the basal series
in the main Cordillera. On the other hand, in the Cordillera, we do not
find those endless varieties of indurated tuffs, with their numerous
veins and concretionary arrangement, and those grit and mud stones, and
singular semi-porcellanic rocks, so abundant in the Uspallata range.
The submarine lavas, also, differ considerably; the feldspathic streams
of the Cordillera contain much mica, which is absent in those of the
Uspallata range: in this latter range we have seen on how grand a
scale, basaltic lava has been poured forth, of which there is not a
trace in the Cordillera. This dissimilarity is the more striking,
considering that these two parallel chains are separated by a plain
only between ten and fifteen miles in width; and that the Uspallata
lavas, as well as no doubt the alternating tufaceous beds, have
proceeded from the west, from points apparently between the two ranges.
To imagine that these two piles of strata were contemporaneously
deposited in two closely adjoining, very deep, submarine areas,
separated from each other by a lofty ridge, where a plain now extends,
would be a gratuitous hypothesis. And had they been contemporaneously
deposited, without any such dividing ridge, surely some of the gypseous
and other sedimentary matter forming such immensely thick masses in the
Cordillera, would have extended this short distance eastwards; and
surely some of the Uspallata tuffs and basalts also accumulated to so
great a thickness, would have extended a little westward. Hence I
conclude, that it is far from probable that these two series are not
contemporaneous; but that the strata of one of the chains were
deposited, and even the chain itself uplifted, before the formation of
the other:—which chain, then, is the oldest? Considering that in the
Uspallata range the lowest strata on the western flank lie
unconformably on the clay- slate, as probably is the case with those on
the eastern flank, whereas in the Cordillera all the overlying strata
lie conformably on this formation:—considering that in the Uspallata
range some of the beds, both low down and high up in the series, are
marked with vegetable impressions, showing the continued existence of
neighbouring land;—considering the close general resemblance between
the deposits of this range and those of tertiary origin in several
parts of the continent;—and lastly, even considering the lesser height
and outlying position of the Uspallata range,—I conclude that the
strata composing it are in all probability of subsequent origin, and
that they were accumulated at a period when a deep sea studded with
submarine volcanoes washed the eastern base of the already partially
elevated Cordillera.
This conclusion is of much importance, for we have seen that in the
Cordillera, during the deposition of the Neocomian strata, the bed of
the sea must have subsided many thousand feet: we now learn that at a
later period an adjoining area first received a great accumulation of
strata, and was upheaved into land on which coniferous trees grew, and
that this area then subsided several thousand feet to receive the
superincumbent submarine strata, afterwards being broken up, denuded,
and elevated in mass to its present height. I am strengthened in this
conclusion of there having been two distinct, great periods of
subsidence, by reflecting on the thick mass of coarse stratified
conglomerate in the valley of Tenuyan, between the Peuquenes and
Portillo lines; for the accumulation of this mass seems to me, as
previously remarked, almost necessarily to have required a prolonged
subsidence; and this subsidence, from the pebbles in the conglomerate
having been to a great extent derived from the gypseous or Neocomian
strata of the Peuquenes line, we know must have been quite distinct
from, and subsequent to, that sinking movement which probably
accompanied the deposition of the Peuquenes strata, and which certainly
accompanied the deposition of the equivalent beds near the Puente del
Inca, in this line of section.
The Uspallata chain corresponds in geographical position, though on a
small scale, with the Portillo line; and its clay-slate formation is
probably the equivalent of the mica-schist of the Portillo, there
metamorphosed by the old white granites and syenites. The coloured beds
under the conglomerate in the valley of Tenuyan, of which traces are
seen on the crest of the Portillo, and even the conglomerate itself,
may perhaps be synchronous with the tufaceous beds and submarine lavas
of the Uspallata range; an open sea and volcanic action in the latter
case, and a confined channel between two bordering chains of islets in
the former case, having been sufficient to account for the
mineralogical dissimilarity of the two series. From this correspondence
between the Uspallata and Portillo ranges, perhaps in age and certainly
in geographical position, one is tempted to consider the one range as
the prolongation of the other; but their axes are formed of totally
different intrusive rocks; and we have traced the apparent continuation
of the red granite of the Portillo in the red porphyries diverging into
the main Cordillera. Whether the axis of the Uspallata range was
injected before, or as perhaps is more probable, after that of the
Portillo line, I will not pretend to decide; but it is well to remember
that the highly inclined lava-streams on the eastern flank of the
Portillo line, prove that its angular upheavement was not a single and
sudden event; and therefore that the anticlinal elevation of the
Uspallata range may have been contemporaneous with some of the later
angular movements by which the gigantic Portillo range gained its
present height above the adjoining plain.
CHAPTER VIII.
NORTHERN CHILE. CONCLUSION.
Section from Illapel to Combarbala; gypseous formation with silicified
wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up valley;
fossils.—Guasco, fossils of.—Copiapo, section up valley; Las Amolanas,
silicified wood.—Conglomerates, nature of former land, fossils,
thickness of strata, great subsidence.—Valley of Despoblado, fossils,
tufaceous deposit, complicated dislocations of.—Relations between
ancient orifices of eruption and subsequent axes of injection.—Iquique,
Peru, fossils of, salt-deposits.—Metalliferous veins.—Summary on the
porphyritic conglomerate and gypseous formations.—Great subsidence with
partial elevations during the cretaceo-oolitic period.—On the elevation
and structure of the Cordillera.—Recapitulation on the tertiary
series.—Relation between movements of subsidence and volcanic
action.—Pampean formation.—Recent elevatory movements.—Long-continued
volcanic action in the Cordillera.—Conclusion.
VALPARAISO TO COQUIMBO.
I have already described the general nature of the rocks in the low
country north of Valparaiso, consisting of granites, syenites,
greenstones, and altered feldspathic clay-slate. Near Coquimbo there is
much hornblendic rock and various dusky-coloured porphyries. I will
describe only one section in this district, namely, from near Illapel
in a N.E. line to the mines of Los Hornos, and thence in a north by
east direction to Combarbala, at the foot of the main Cordillera.
Near Illapel, after passing for some distance over granite, andesite,
and andesitic porphyry, we come to a greenish stratified feldspathic
rock, which I believe is altered clay-slate, conformably capped by
porphyries and porphyritic conglomerate of great thickness, dipping at
an average angle of 20 degrees to N.E. by N. The uppermost beds consist
of conglomerates and sandstone only a little metamorphosed, and
conformably covered by a gypseous formation of very great thickness,
but much denuded. This gypseous formation, where first met with, lies
in a broad valley or basin, a little southward of the mines of Los
Hornos: the lower half alone contains gypsum, not in great masses as in
the Cordillera, but in innumerable thin layers, seldom more than an
inch or two in thickness. The gypsum is either opaque or transparent,
and is associated with carbonate of lime. The layers alternate with
numerous varying ones of a calcareous clay-shale (with strong aluminous
odour, adhering to the tongue, easily fusible into a pale green glass),
more or less indurated, either earthy and cream-coloured, or greenish
and hard. The more indurated varieties have a compact, homogeneous,
almost crystalline fracture, and contain granules of crystallised oxide
of iron. Some of the varieties almost resemble honestones. There is
also a little black, hardly fusible, siliceo- calcareous clay-slate,
like some of the varieties alternating with gypsum on the Peuquenes
range.
The upper half of this gypseous formation is mainly formed of the same
calcareous clay-shale rock, but without any gypsum, and varying
extremely in nature: it passes from a soft, coarse, earthy, ferruginous
state, including particles of quartz, into compact claystones with
crystallised oxide of iron,—into porcellanic layers, alternating with
seams of calcareous matter,—and into green porcelain-jasper,
excessively hard, but easily fusible. Strata of this nature alternate
with much black and brown siliceo-calcareous slate, remarkable from the
wonderful number of huge embedded logs of silicified wood. This wood,
according to Mr. R. Brown, is (judging from several specimens) all
coniferous. Some of the layers of the black siliceous slate contained
irregular angular fragments of imperfect pitchstone, which I believe,
as in the Uspallata range, has originated in a metamorphic process.
There was one bed of a marly tufaceous nature, and of little specific
gravity. Veins of agate and calcareous spar are numerous. The whole of
this gypseous formation, especially the upper half, has been injected,
metamorphosed, and locally contorted by numerous hillocks of intrusive
porphyries crowded together in an extraordinary manner. These hillocks
consist of purple claystone and of various other porphyries, and of
much white feldspathic greenstone passing into andesite; this latter
variety included in one case crystals of orthitic and albitic feldspar
touching each other, and others of hornblende, chlorite, and epidote.
The strata surrounding these intrusive hillocks at the mines of Los
Hornos, are intersected by many veins of copper-pyrites, associated
with much micaceous iron-ore, and by some of gold: in the neighbourhood
of these veins the rocks are blackened and much altered. The gypsum
near the intrusive masses is always opaque. One of these hillocks of
porphyry was capped by some stratified porphyritic conglomerate, which
must have been brought up from below, through the whole immense
thickness of the overlying gypseous formation. The lower beds of the
gypseous formation resemble the corresponding and probably
contemporaneous strata of the main Cordillera; whilst the upper beds in
several respects resemble those of the Uspallata chain, and possibly
may be contemporaneous with them; for I have endeavoured to show that
the Uspallata beds were accumulated subsequently to the gypseous or
Neocomian formations of the Cordillera.
This pile of strata dips at an angle of about 20 degrees to N.E. by N.,
close up to the foot of the Cuesta de Los Hornos, a crooked range of
mountains formed of intrusive rocks of the same nature with the above
described hillocks. Only in one or two places, on this south-eastern
side of the range, I noticed a narrow fringe of the upper gypseous
strata brushed up and inclined south-eastward from it. On its
north-eastern flank, and likewise on a few of the summits, the
stratified porphyritic conglomerate is inclined N.E.: so that, if we
disregard the very narrow anticlinal fringe of gypseous strata at its
S.E. foot, this range forms a second uniclinal axis of elevation.
Proceeding in a north-by-east direction to the village of Combarbala,
we come to a third escarpment of the porphyritic conglomerate, dipping
eastwards, and forming the outer range of the main Cordillera. The
lower beds were here more jaspery than usual, and they included some
white cherty strata and red sandstones, alternating with purple
claystone porphyry. Higher up in the Cordillera there appeared to be a
line of andesitic rocks; and beyond them, a fourth escarpment of the
porphyritic conglomerate, again dipping eastwards or inwards. The
overlying gypseous strata, if they ever existed here, have been
entirely removed.
COPPER MINES OF PANUNCILLO.
From Combarbala to Coquimbo, I traversed the country in a zigzag
direction, crossing and recrossing the porphyritic conglomerate and
finding in the granitic districts an unusual number of mountain-masses
composed of various intrusive, porphyritic rocks, many of them
andesitic. One common variety was greenish-black, with large crystals
of blackish albite. At Panuncillo a short N.N.W. and S.S.E. ridge, with
a nucleus formed of greenstone and of a slate-coloured porphyry
including crystals of glassy feldspar, deserves notice, from the very
singular nature of the almost vertical strata composing it. These
consist chiefly of a finer and coarser granular mixture, not very
compact, of white carbonate of lime, of protoxide of iron and of
yellowish garnets (ascertained by Professor Miller), each grain being
an almost perfect crystal. Some of the varieties consist exclusively of
granules of the calcareous spar; and some contain grains of copper ore,
and, I believe, of quartz. These strata alternate with a bluish,
compact, fusible, feldspathic rock. Much of the above granular mixture
has, also, a pseudo-brecciated structure, in which fragments are
obscurely arranged in planes parallel to those of the stratification,
and are conspicuous on the weathered surfaces. The fragments are
angular or rounded, small or large, and consist of bluish or reddish
compact feldspathic matter, in which a few acicular crystals of
feldspar can sometimes be seen. The fragments often blend at their
edges into the surrounding granular mass, and seem due to a kind of
concretionary action.
These singular rocks are traversed by many copper veins, and appear to
rest conformably on the granular mixture (in parts as fine-grained as a
sandstone) of quartz, mica, hornblende, and feldspar; and this on fine-
grained, common gneiss; and this on a laminated mass, composed of
pinkish ORTHITIC feldspar, including a few specks of hornblende; and
lastly, this on granite, which together with andesitic rocks, form the
surrounding district.
COQUIMBO: MINING DISTRICT OF ARQUEROS.
At Coquimbo the porphyritic conglomerate formation approaches nearer to
the Pacific than in any other part of Chile visited by me, being
separated from the coast by a tract only a few miles broad of the usual
plutonic rocks, with the addition of a porphyry having a red euritic
base. In proceeding to the mines of Arqueros, the strata of porphyritic
conglomerate are at first nearly horizontal, an unusual circumstance,
and afterwards they dip gently to S.S.E. After having ascended to a
considerable height, we come to an undulatory district in which the
famous silver mines are situated; my examination was chiefly confined
to those of S. Rosa. Most of the rocks in this district are stratified,
dipping in various directions, and many of them are of so singular a
nature, that at the risk of being tedious I must briefly describe them.
The commonest variety is a dull-red, compact, finely brecciated stone,
containing much iron and innumerable white crystallised particles of
carbonate of lime, and minute extraneous fragments. Another variety is
almost equally common near S. Rosa; it has a bright green, scanty
basis, including distinct crystals and patches of white carbonate of
lime, and grains of red, semi-micaceous oxide of iron; in parts the
basis becomes dark green, and assumes an obscure crystalline
arrangement, and occasionally in parts it becomes soft and slightly
translucent like soapstone. These red and green rocks are often quite
distinct, and often pass into each other; the passage being sometimes
affected by a fine brecciated structure, particles of the red and green
matter being mingled together. Some of the varieties appear gradually
to become porphyritic with feldspar; and all of them are easily fusible
into pale or dark-coloured beads, strongly attracted by the magnet. I
should perhaps have mistaken several of these stratified rocks for
submarine lavas, like some of those described at the Puente del Inca,
had I not examined, a few leagues eastward of this point, a fine series
of analogous but less metamorphosed, sedimentary beds belonging to the
gypseous formation, and probably derived from a volcanic source.
This formation is intersected by numerous metalliferous veins, running,
though irregularly, N.W. and S.E., and generally at right angles to the
many dikes. The veins consist of native silver, of muriate of silver,
an amalgam of silver, cobalt, antimony, and arsenic, generally embedded
in sulphate of barytes. (See the Report on M. Domeyko’s account of
those mines, in the “Comptes Rendus” tome 14 page 560.) I was assured
by Mr. Lambert, that native copper without a trace of silver has been
found in the same vein with native silver without a trace of copper. At
the mines of Aristeas, the silver veins are said to be unproductive as
soon as they pass into the green strata, whereas at S. Rosa, only two
or three miles distant, the reverse happens; and at the time of my
visit, the miners were working through a red stratum, in the hope of
the vein becoming productive in the underlying green sedimentary mass.
I have a specimen of one of these green rocks, with the usual granules
of white calcareous spar and red oxide of iron, abounding with
disseminated particles of glittering native and muriate of silver, yet
taken at the distance of one yard from any vein,—a circumstance, as I
was assured, of very rare occurrence.
A SECTION EASTWARD, UP THE VALLEY OF COQUIMBO.
After passing for a few miles over the coast granitic series, we come
to the porphyritic conglomerate, with its usual characters, and with
some of the beds distinctly displaying their mechanical origin. The
strata, where first met with, are, as before stated, only slightly
inclined; but near the Hacienda of Pluclaro, we come to an anticlinal
axis, with the beds much dislocated and shifted by a great fault, of
which not a trace is externally seen in the outline of the hill. I
believe that this anticlinal axis can be traced northwards, into the
district of Arqueros, where a conspicuous hill called Cerro Blanco,
formed of a harsh, cream-coloured euritic rock, including a few
crystals of reddish feldspar, and associated with some purplish
claystone porphyry, seems to fall on a line of elevation. In descending
from the Arqueros district, I crossed on the northern border of the
valley, strata inclined eastward from the Pluclaro axis: on the
porphyritic conglomerate there rested a mass, some hundred feet thick,
of brown argillaceous limestone, in parts crystalline, and in parts
almost composed of Hippurites Chilensis, d’Orbigny; above this came a
black calcareous shale, and on it a red conglomerate. In the brown
limestone, with the Hippurites, there was an impression of a Pecten and
a coral, and great numbers of a large Gryphaea, very like, and,
according to Professor E. Forbes, probably identical with G.
Orientalis, Forbes MS.,—a cretaceous species (probably upper greensand)
from Verdachellum, in Southern India. These fossils seem to occupy
nearly the same position with those at the Puente del Inca,—namely, at
the top of the porphyritic conglomerate, and at the base of the
gypseous formation.
A little above the Hacienda of Pluclaro, I made a detour on the
northern side of the valley, to examine the superincumbent gypseous
strata, which I estimated at 6,000 feet in thickness. The uppermost
beds of the porphyritic conglomerate, on which the gypseous strata
conformably rest, are variously coloured, with one very singular and
beautiful stratum composed of purple pebbles of various kinds of
porphyry, embedded in white calcareous spar, including cavities lined
with bright-green crystallised epidote. The whole pile of strata
belonging to both formations is inclined, apparently from the
above-mentioned axis of Pluclaro, at an angle of between 20 and 30
degrees to the east. I will here give a section of the principal beds
met with in crossing the entire thickness of the gypseous strata.
Firstly: above the porphyritic conglomerate formation, there is a fine-
grained, red, crystalline sandstone.
Secondly: a thick mass of smooth-grained, calcareo-aluminous, shaly
rock, often marked with dendritic manganese, and having, where most
compact, the external appearance of honestone. It is easily fusible. I
shall for the future, for convenience’ sake, call this variety
pseudo-honestone. Some of the varieties are quite black when freshly
broken, but all weather into a yellowish-ash coloured, soft, earthy
substance, precisely as is the case with the compact shaly rocks of the
Peuquenes range. This stratum is of the same general nature with many
of the beds near Los Hornos in the Illapel section. In this second bed,
or in the underlying red sandstone (for the surface was partially
concealed by detritus), there was a thick mass of gypsum, having the
same mineralogical characters with the great beds described in our
sections across the Cordillera.
Thirdly: a thick stratum of fine-grained, red, sedimentary matter,
easily fusible into a white glass, like the basis of claystone
porphyry; but in parts jaspery, in parts brecciated, and including
crystalline specks of carbonate of lime. In some of the jaspery layers,
and in some of the black siliceous slaty bands, there were irregular
seams of imperfect pitchstone, undoubtedly of metamorphic origin, and
other seams of brown, crystalline limestone. Here, also, were masses,
externally resembling ill-preserved silicified wood.
Fourthly and fifthly: calcareous pseudo-honestone; and a thick stratum
concealed by detritus.
Sixthly: a thinly stratified mass of bright green, compact,
smooth-grained, calcareo-argillaceous stone, easily fusible, and
emitting a strong aluminous odour: the whole has a highly
angulo-concretionary structure; and it resembles, to a certain extent,
some of the upper tufaceo-infusorial deposits of the Patagonian
tertiary formation. It is in its nature allied to our pseudo-honestone,
and it includes well characterised layers of that variety; and other
layers of a pale green, harder, and brecciated variety; and others of
red sedimentary matter, like that of bed Three. Some pebbles of
porphyries are embedded in the upper part.
Seventhly: red sedimentary matter or sandstone like that of bed One,
several hundred feet in thickness, and including jaspery layers, often
having a finely brecciated structure.
Eighthly: white, much indurated, almost crystalline tuff, several
hundred feet in thickness, including rounded grains of quartz and
particles of green matter like that of bed Six. Parts pass into a very
pale green, semi- porcellanic stone.
Ninthly: red or brown coarse conglomerate, three or four hundred feet
thick, formed chiefly of pebbles of porphyries, with volcanic
particles, in an arenaceous, non-calcareous, fusible basis: the upper
two feet are arenaceous without any pebbles.
Tenthly: the last and uppermost stratum here exhibited, is a compact,
slate-coloured porphyry, with numerous elongated crystals of glassy
feldspar, from one hundred and fifty to two hundred feet in thickness;
it lies strictly conformably on the underlying conglomerate, and is
undoubtedly a submarine lava.
This great pile of strata has been broken up in several places by
intrusive hillocks of purple claystone porphyry, and by dikes of
porphyritic greenstone: it is said that a few poor metalliferous veins
have been discovered here. From the fusible nature and general
appearance of the finer-grained strata, they probably owe their origin
(like the allied beds of the Uspallata range, and of the Upper
Patagonian tertiary formations), to gentle volcanic eruptions, and to
the abrasion of volcanic rocks. Comparing these beds with those in the
mining district of Arqueros, we see at both places rocks easily
fusible, of the same peculiar bright green and red colours, containing
calcareous matter, often having a finely brecciated structure, often
passing into each other, and often alternating together: hence I cannot
doubt that the only difference between them, lies in the Arqueros beds
having been more metamorphosed (in conformity with their more
dislocated and injected condition), and consequently in the calcareous
matter, oxide of iron and green colouring matter, having been
segregated under a more crystalline form.
The strata are inclined, as before stated, from 20 to 30 degrees
eastward, towards an irregular north and south chain of andesitic
porphyry and of porphyritic greenstone, where they are abruptly cut
off. In the valley of Coquimbo, near to the H. of Gualliguaca, similar
plutonic rocks are met with, apparently a southern prolongation of the
above chain; and eastward of it we have an escarpment of the
porphyritic conglomerate, with the strata inclined at a small angle
eastward, which makes the third escarpment, including that nearest the
coast. Proceeding up the valley we come to another north and south line
of granite, andesite, and blackish porphyry, which seem to lie in an
irregular trough of the porphyritic conglomerate. Again, on the south
side of the R. Claro, there are some irregular granitic hills, which
have thrown off the strata of porphyritic conglomerate to the N.W. by
W.; but the stratification here has been much disturbed. I did not
proceed any farther up the valley, and this point is about two-thirds
of the distance between the Pacific and the main Cordillera.
I will describe only one other section, namely, on the north side of
the R. Claro, which is interesting from containing fossils: the strata
are much dislocated by faults and dikes, and are inclined to the north,
towards a mountain of andesite and porphyry, into which they appear to
become almost blended. As the beds approach this mountain, their
inclination increases up to an angle of 70 degrees, and in the upper
part, the rocks become highly metamorphosed. The lowest bed visible in
this section, is a purplish hard sandstone. Secondly, a bed two or
three hundred feet thick, of a white siliceous sandstone, with a
calcareous cement, containing seams of slaty sandstone, and of hard
yellowish-brown (dolomitic?) limestone; numerous, well-rounded, little
pebbles of quartz are included in the sandstone. Thirdly, a dark
coloured limestone with some quartz pebbles, from fifty to sixty feet
in thickness, containing numerous silicified shells, presently to be
enumerated. Fourthly, very compact, calcareous, jaspery sandstone,
passing into (fifthly) a great bed, several hundred feet thick, of
conglomerate, composed of pebbles of white, red, and purple porphyries,
of sandstone and quartz, cemented by calcareous matter. I observed that
some of the finer parts of this conglomerate were much indurated within
a foot of a dike eight feet in width, and were rendered of a paler
colour with the calcareous matter segregated into white crystallised
particles; some parts were stained green from the colouring matter of
the dike. Sixthly, a thick mass, obscurely stratified, of a red
sedimentary stone or sandstone, full of crystalline calcareous matter,
imperfect crystals of oxide of iron, and I believe of feldspar, and
therefore closely resembling some of the highly metamorphosed beds at
Arqueros: this bed was capped by, and appeared to pass in its upper
part into, rocks similarly coloured, containing calcareous matter, and
abounding with minute crystals, mostly elongated and glassy, of reddish
albite. Seventhly, a conformable stratum of fine reddish porphyry with
large crystals of (albitic?) feldspar; probably a submarine lava.
Eighthly, another conformable bed of green porphyry, with specks of
green earth and cream-coloured crystals of feldspar. I believe that
there are other superincumbent crystalline strata and submarine lavas,
but I had not time to examine them.
The upper beds in this section probably correspond with parts of the
great gypseous formation; and the lower beds of red sandstone
conglomerate and fossiliferous limestone no doubt are the equivalents
of the Hippurite stratum, seen in descending from Arqueros to Pluclaro,
which there lies conformably upon the porphyritic conglomerate
formation. The fossils found in the third bed, consist of:—
Pecten Dufreynoyi, d’Orbigny, “Voyage, Part Pal.” This species, which
occurs here in vast numbers, according to M. D’Orbigny, resembles
certain cretaceous forms.
Ostrea hemispherica, d’Orbigny, “Voyage” etc.
Also resembles, according to the same author, cretaceous forms.
Terebratula aenigma, d’Orbigny, “Voyage” etc. (Pl. 22 Figures 10-12.)
Is allied, according to M. d’Orbigny, to T. concinna from the Forest
Marble. A series of this species, collected in several localities
hereafter to be referred to, has been laid before Professor Forbes; and
he informs me that many of the specimens are almost undistinguishable
from our oolitic T. tetraedra, and that the varieties amongst them are
such as are found in that variable species. Generally speaking, the
American specimens of T. aenigma may be distinguished from the British
T. tetraedra, by the surface having the ribs sharp and well-defined to
the beak, whilst in the British species they become obsolete and
smoothed down; but this difference is not constant. Professor Forbes
adds, that, possibly, internal characters may exist, which would
distinguish the American species from its European allies.
Spirifer linguiferoides, E. Forbes.
Professor Forbes states that this species is very near to S. linguifera
of Phillips (a carboniferous limestone fossil), but probably distinct.
M. d’Orbigny considers it as perhaps indicating the Jurassic period.
Ammonites, imperfect impression of.
M. Domeyko has sent to France a collection of fossils, which, I
presume, from the description given, must have come from the
neighbourhood of Arqueros; they consist of:—
Pecten Dufreynoyi, d’Orbigny, “Voyage” Part Pal. Ostrea hemispherica,
d’Orbigny, “Voyage” Part Pal. Turritella Andii, d’Orbigny, “Voyage”
Part Pal. (Pleurotomaria Humboldtii of Von Buch). Hippurites Chilensis,
d’Orbigny, “Voyage” Part Pal.
The specimens of this Hippurite, as well as those I collected in my
descent from Arqueros, are very imperfect; but in M. d’Orbigny’s
opinion they resemble, as does the Turritella Andii, cretaceous (upper
greensand) forms.
Nautilus Domeykus, d’Orbigny, “Voyage” Part Pal. Terebratula aenigma,
d’Orbigny, “Voyage” Part Pal. Terebratula ignaciana, d’Orbigny,
“Voyage” Part Pal.
This latter species was found by M. Domeyko in the same block of
limestone with the T. aenigma. According to M. d’Orbigny, it comes near
to T. ornithocephala from the Lias. A series of this species collected
at Guasco, has been examined by Professor E. Forbes, and he states that
it is difficult to distinguish between some of the specimens and the T.
hastata from the mountain limestone; and that it is equally difficult
to draw a line between them and some Marlstone Terebratulae. Without a
knowledge of the internal structure, it is impossible at present to
decide on their identity with analogous European forms.
The remarks given on the several foregoing shells, show that, in M.
d’Orbigny’s opinion, the Pecten, Ostrea, Turritella, and Hippurite
indicate the cretaceous period; and the Gryphaea appears to Professor
Forbes to be identical with a species, associated in Southern India
with unquestionably cretaceous forms. On the other hand, the two
Terebratulae and the Spirifer point, in the opinion both of M.
d’Orbigny and Professor Forbes, to the oolitic series. Hence M.
d’Orbigny, not having himself examined this country, has concluded that
there are here two distinct formations; but the Spirifer and T. aenigma
were certainly included in the same bed with the Pecten and Ostrea,
whence I extracted them; and the geologist M. Domeyko sent home the two
Terebratulae with the other-named shells, from the same locality,
without specifying that they came from different beds. Again, as we
shall presently see, in a collection of shells given me from Guasco,
the same species, and others presenting analogous differences, are
mingled together, and are in the same condition; and lastly, in three
places in the valley of Copiapo, I found some of these same species
similarly grouped. Hence there cannot be any doubt, highly curious
though the fact be, that these several fossils, namely, the Hippurites,
Gryphaea, Ostrea, Pecten, Turritella, Nautilus, two Terebratulae, and
Spirifer all belong to the same formation, which would appear to form a
passage between the oolitic and cretaceous systems of Europe. Although
aware how unusual the term must sound, I shall, for convenience’ sake,
call this formation cretaceo- oolitic. Comparing the sections in this
valley of Coquimbo with those in the Cordillera described in the last
chapter, and bearing in mind the character of the beds in the
intermediate district of Los Hornos, there is certainly a close general
mineralogical resemblance between them, both in the underlying
porphyritic conglomerate, and in the overlying gypseous formation.
Considering this resemblance, and that the fossils from the Puente del
Inca at the base of the gypseous formation, and throughout the greater
part of its entire thickness on the Peuquenes range, indicate the
Neocomian period,—that is, the dawn of the cretaceous system, or, as
some have believed, a passage between this latter and the oolitic
series—I conclude that probably the gypseous and associated beds in all
the sections hitherto described, belong to the same great formation,
which I have denominated—cretaceo-oolitic. I may add, before leaving
Coquimbo, that M. Gay found in the neighbouring Cordillera, at the
height of 14,000 feet above the sea, a fossiliferous formation,
including a Trigonia and Pholadomya (D’Orbigny “Voyage” Part Geolog.
page 242.);—both of which genera occur at the Puente del Inca.
COQUIMBO TO GUASCO.
The rocks near the coast, and some way inland, do not differ from those
described northwards of Valparaiso: we have much greenstone, syenite,
feldspathic and jaspery slate, and grauwackes having a basis like that
of claystone; there are some large tracts of granite, in which the
constituent minerals are sometimes arranged in folia, thus composing an
imperfect gneiss. There are two large districts of mica-schists,
passing into glossy clay-slate, and resembling the great formation in
the Chonos Archipelago. In the valley of Guasco, an escarpment of
porphyritic conglomerate is first seen high up the valley, about two
leagues eastward of the town of Ballenar. I heard of a great gypseous
formation in the Cordillera; and a collection of shells made there was
given me. These shells are all in the same condition, and appear to
have come from the same bed: they consist of:—
Turritella Andii, d’Orbigny, “Voyage” Part Pal. Pecten Dufreynoyi,
d’Orbigny, “Voyage” Part Pal. Terebatula ignaciana, d’Orbigny, “Voyage”
Part Pal.
The relations of these species have been given under the head of
Coquimbo.
Terebratula aenigma, d’Orbigny, “Voyage” Part Pal.
This shell M. d’Orbigny does not consider identical with his T.
aenigma, but near to T. obsoleta. Professor Forbes thinks that it is
certainly a variety of T. aenigma: we shall meet with this variety
again at Copiapo.
Spirifer Chilensis, E. Forbes.
Professor Forbes remarks that this fossil resembles several
carboniferous limestone Spirifers; and that it is also related to some
liassic species, as S. Wolcotii.
If these shells had been examined independently of the other
collections, they would probably have been considered, from the
characters of the two Terebratulae, and from the Spirifer, as oolitic;
but considering that the first species, and according to Professor
Forbes, the four first, are identical with those from Coquimbo, the two
formations no doubt are the same, and may, as I have said, be
provisionally called cretaceo-oolitic.
VALLEY OF COPIAPO.
The journey from Guasco to Copiapo, owing to the utterly desert nature
of the country, was necessarily so hurried, that I do not consider my
notes worth giving. In the valley of Copiapo some of the sections are
very interesting. From the sea to the town of Copiapo, a distance
estimated at thirty miles, the mountains are composed of greenstone,
granite, andesite, and blackish porphyry, together with some
dusky-green feldspathic rocks, which I believe to be altered
clay-slate: these mountains are crossed by many brown-coloured dikes,
running north and south. Above the town, the main valley runs in a
south-east and even more southerly course towards the Cordillera, where
it is divided into three great ravines, by the northern one of which,
called Jolquera, I penetrated for a short distance. The section,
Section 1/3 in Plate 1, gives an eye-sketch of the structure and
composition of the mountains on both sides of this valley: a straight
east and west line from the town to the Cordillera is perhaps not more
than thirty miles, but along the valley the distance is much greater.
Wherever the valley trended very southerly, I have endeavoured to
contract the section into its true proportion. This valley, I may add,
rises much more gently than any other valley which I saw in Chile.
To commence with our section, for a short distance above the town we
have hills of the granitic series, together with some of that rock [A],
which I suspect to be altered clay-slate, but which Professor G. Rose,
judging from specimens collected by Meyen at P. Negro, states is
serpentine passing into greenstone. We then come suddenly to the great
gypseous formation [B], without having passed over, differently from,
in all the sections hitherto described, any of the porphyritic
conglomerate. The strata are at first either horizontal or gently
inclined westward; then highly inclined in various directions, and
contorted by underlying masses of intrusive rocks; and lastly, they
have a regular eastward dip, and form a tolerably well pronounced north
and south line of hills. This formation consists of thin strata, with
innumerable alternations, of black, calcareous slate-rock, of
calcareo-aluminous stones like those at Coquimbo, which I have called
pseudo-honestones of green jaspery layers, and of pale-purplish,
calcareous, soft rotten-stone, including seams and veins of gypsum.
These strata are conformably overlaid by a great thickness of thinly
stratified, compact limestone with included crystals of carbonate of
lime. At a place called Tierra Amarilla, at the foot of a mountain thus
composed there is a broad vein, or perhaps stratum, of a beautiful and
curious crystallised mixture, composed, according to Professor G. Rose,
of sulphate of iron under two forms, and of the sulphates of copper and
alumina (Meyen’s “Reise” etc. Th. 1, s. 394.): the section is so
obscure that I could not make out whether this vein or stratum occurred
in the gypseous formation, or more probably in some underlying masses
[A], which I believe are altered clay-slate.
SECOND AXIS OF ELEVATION.
After the gypseous masses [B], we come to a line of hills of
unstratified porphyry [C], which on their eastern side blend into
strata of great thickness of porphyritic conglomerate, dipping
eastward. This latter formation, however, here has not been nearly so
much metamorphosed as in most parts of Central Chile; it is composed of
beds of true purple claystone porphyry, repeatedly alternating with
thick beds of purplish-red conglomerate with the well-rounded, large
pebbles of various porphyries, not blended together.
THIRD AXIS OF ELEVATION.
Near the ravine of Los Hornitos, there is a well-marked line of
elevation, extending for many miles in a N.N.E. and S.S.W. direction,
with the strata dipping in most parts (as in the second axis) only in
one direction, namely, eastward at an average angle of between 30 and
40 degrees. Close to the mouth of the valley, however, there is, as
represented in the section, a steep and high mountain [D], composed of
various green and brown intrusive porphyries enveloped with strata,
apparently belonging to the upper parts of the porphyritic
conglomerate, and dipping both eastward and westward. I will describe
the section seen on the eastern side of this mountain [D], beginning at
the base with the lowest bed visible in the porphyritic conglomerate,
and proceeding upwards through the gypseous formation. Bed 1 consists
of reddish and brownish porphyry varying in character, and in many
parts highly amygdaloidal with carbonate of lime, and with bright green
and brown bole. Its upper surface is throughout clearly defined, but
the lower surface is in most parts indistinct, and towards the summit
of the mountain [D] quite blended into the intrusive porphyries. Bed 2,
a pale lilac, hard but not heavy stone, slightly laminated, including
small extraneous fragments, and imperfect as well as some perfect and
glassy crystals of feldspar; from one hundred and fifty to two hundred
feet in thickness. When examining it in situ, I thought it was
certainly a true porphyry, but my specimens now lead me to suspect that
it possibly may be a metamorphosed tuff. From its colour it could be
traced for a long distance, overlying in one part, quite conformably to
the porphyry of bed 1, and in another not distant part, a very thick
mass of conglomerate, composed of pebbles of a porphyry chiefly like
that of bed 1: this fact shows how the nature of the bottom formerly
varied in short horizontal distances. Bed 3, white, much indurated
tuff, containing minute pebbles, broken crystals, and scales of mica,
varies much in thickness. This bed is remarkable from containing many
globular and pear-shaped, externally rusty balls, from the size of an
apple to a man’s head, of very tough, slate-coloured porphyry, with
imperfect crystals of feldspar: in shape these balls do not resemble
pebbles, AND I BELIEVE THAT THEY ARE SUBAQUEOUS VOLCANIC BOMBS; they
differ from SUBAERIAL bombs only in not being vesicular. Bed 4; a dull
purplish-red, hard conglomerate, with crystallised particles and veins
of carbonate of lime, from three hundred to four hundred feet in
thickness. The pebbles are of claystone porphyries of many varieties;
they are tolerably well rounded, and vary in size from a large apple to
a man’s head. This bed includes three layers of coarse, black,
calcareous, somewhat slaty rock: the upper part passes into a compact
red sandstone.
In a formation so highly variable in mineralogical nature, any division
not founded on fossil remains, must be extremely arbitrary:
nevertheless, the beds below the last conglomerate may, in accordance
with all the sections hitherto described, be considered as belonging to
the porphyritic conglomerate, and those above it to the gypseous
formation, marked [E] in the section. The part of the valley in which
the following beds are seen is near Potrero Seco. Bed 5, compact,
fine-grained, pale greenish-grey, non- calcareous, indurated mudstone,
easily fusible into a pale green and white glass. Bed 6, purplish,
coarse-grained, hard sandstone, with broken crystals of feldspar and
crystallised particles of carbonate of lime; it possesses a slightly
nodular structure. Bed 7, blackish-grey, much indurated, calcareous
mudstone, with extraneous particles of unequal size; the whole being in
parts finely brecciated. In this mass there is a stratum, twenty feet
in thickness, of impure gypsum. Bed 8, a greenish mudstone, with
several layers of gypsum. Bed 9, a highly indurated, easily fusible,
white tuff, thickly mottled with ferruginous matter, and including some
white semi-porcellanic layers, which are interlaced with ferruginous
veins. This stone closely resembles some of the commonest varieties in
the Uspallata chain. Bed 10, a thick bed of rather bright green,
indurated mudstone or tuff, with a concretionary nodular structure so
strongly developed that the whole mass consists of balls. I will not
attempt to estimate the thickness of the strata in the gypseous
formation hitherto described, but it must certainly be very many
hundred feet. Bed 11 is at least 800 feet in thickness: it consists of
thin layers of whitish, greenish, or more commonly brown, fine-grained,
indurated tuffs, which crumble into angular fragments: some of the
layers are semi-porcellanic, many of them highly ferruginous, and some
are almost composed of carbonate of lime and iron with drusy cavities
lined with quartzf-crystals. Bed 12, dull purplish or greenish or
dark-grey, very compact and much indurated mudstone: estimated at 1,500
feet in thickness: in some parts this rock assumes the character of an
imperfect coarse clay-slate; but viewed under a lens, the basis always
has a mottled appearance, with the edges of the minute component
particles blending together. Parts are calcareous, and there are
numerous veins of highly crystalline carbonate of lime charged with
iron. The mass has a nodular structure, and is divided by only a few
planes of stratification: there are, however, two layers, each about
eighteen inches thick, of a dark brown, finer-grained stone, having a
conchoidal, semi-porcellanic fracture, which can be followed with the
eye for some miles across the country.
I believe this last great bed is covered by other nearly similar
alternations; but the section is here obscured by a tilt from the next
porphyritic chain, presently to be described. I have given this section
in detail, as being illustrative of the general character of the
mountains in this neighbourhood; but it must not be supposed that any
one stratum long preserves the same character. At a distance of between
only two and three miles the green mudstones and white indurated tuffs
are to a great extent replaced by red sandstone and black calcareous
shaly rocks, alternating together. The white indurated tuff, bed 11,
here contains little or no gypsum, whereas on the northern and opposite
side of the valley, it is of much greater thickness and abounds with
layers of gypsum, some of them alternating with thin seams of
crystalline carbonate of lime. The uppermost, dark-coloured, hard
mudstone, bed 12, is in this neighbourhood the most constant stratum.
The whole series differs to a considerable extent, especially in its
upper part, from that met with at [BB], in the lower part of the
valley; nevertheless, I do not doubt that they are equivalents.
FOURTH AXIS OF ELEVATION (VALLEY OF COPIAPO).
This axis is formed of a chain of mountains [F], of which the central
masses (near La Punta) consist of andesite containing green hornblende
and coppery mica, and the outer masses of greenish and black
porphyries, together with some fine lilac-coloured claystone porphyry;
all these porphyries being injected and broken up by small hummocks of
andesite. The central great mass of this latter rock, is covered on the
eastern side by a black, fine-grained, highly micaceous slate, which,
together with the succeeding mountains of porphyry, are traversed by
numerous white dikes, branching from the andesite, and some of them
extending in straight lines, to a distance of at least two miles. The
mountains of porphyry eastward of the micaceous schist soon, but
gradually, assume (as observed in so many other cases) a stratified
structure, and can then be recognised as a part of the porphyritic
conglomerate formation. These strata [G] are inclined at a high angle
to the S.E., and form a mass from fifteen hundred to two thousand feet
in thickness. The gypseous masses to the west already described, dip
directly towards this axis, with the strata only in a few places (one
of which is represented in the section) thrown from it: hence this
fourth axis is mainly uniclinal towards the S.E., and just like our
third axis, only locally anticlinal.
The above strata of porphyritic conglomerate [G] with their
south-eastward dip, come abruptly up against beds of the gypseous
formation [H], which are gently, but irregularly, inclined westward: so
that there is here a synclinal axis and great fault. Further up the
valley, here running nearly north and south, the gypseous formation is
prolonged for some distance; but the stratification is unintelligible,
the whole being broken up by faults, dikes, and metalliferous veins.
The strata consist chiefly of red calcareous sandstones, with numerous
veins in the place of layers, of gypsum; the sandstone is associated
with some black calcareous slate-rock, and with green
pseudo-honestones, passing into porcelain-jasper. Still further up the
valley, near Las Amolanas [I], the gypseous strata become more regular,
dipping at an angle of between 30 and 40 degrees to W.S.W., and
conformably overlying, near the mouth of the ravine of Jolquera, strata
[K] of porphyritic conglomerate. The whole series has been tilted by a
partially concealed axis [L], of granite, andesite, and a granitic
mixture of white feldspar, quartz, and oxide of iron.
FIFTH AXIS OF ELEVATION (VALLEY OF COPIAPO, NEAR LOS AMOLANAS).
I will describe in some detail the beds [I] seen here, which, as just
stated, dip to W.S.W., at an angle of from 30 to 40 degrees. I had not
time to examine the underlying porphyritic conglomerate, of which the
lowest beds, as seen at the mouth of the Jolquera, are highly compact,
with crystals of red oxide of iron; and I am not prepared to say
whether they are chiefly of volcanic or metamorphic origin. On these
beds there rests a coarse purplish conglomerate, very little
metamorphosed, composed of pebbles of porphyry, but remarkable from
containing one pebble of granite;—of which fact no instance has
occurred in the sections hitherto described. Above this conglomerate,
there is a black siliceous claystone, and above it numerous
alternations of dark-purplish and green porphyries, which may be
considered as the uppermost limit of the porphyritic conglomerate
formation.
Above these porphyries comes a coarse, arenaceous conglomerate, the
lower half white and the upper half of a pink colour, composed chiefly
of pebbles of various porphyries, but with some of red sandstone and
jaspery rocks. In some of the more arenaceous parts of the
conglomerate, there was an oblique or current lamination; a
circumstance which I did not elsewhere observe. Above this
conglomerate, there is a vast thickness of thinly stratified,
pale-yellowish, siliceous sandstone, passing into a granular
quartz-rock, used for grindstones (hence the name of the place Las
Amolanas), and certainly belonging to the gypseous formation, as does
probably the immediately underlying conglomerate. In this yellowish
sandstone there are layers of white and pale-red siliceous
conglomerate; other layers with small, well-rounded pebbles of white
quartz, like the bed at the R. Claro at Coquimbo; others of a greenish,
fine-grained, less siliceous stone, somewhat resembling the
pseudo-honestones lower down the valley; and lastly, others of a black
calcareous shale-rock. In one of the layers of conglomerate, there was
embedded a fragment of mica-slate, of which this is the first instance;
hence perhaps, it is from a formation of mica-slate, that the numerous
small pebbles of quartz, both here and at Coquimbo, have been derived.
Not only does the siliceous sandstone include layers of the black,
thinly stratified, not fissile, calcareous shale-rock, but in one place
the whole mass, especially the upper part, was, in a marvellously short
horizontal distance, after frequent alternations, replaced by it. When
this occurred, a mountain-mass, several thousand feet in thickness was
thus composed; the black calcareous shale-rock, however, always
included some layers of the pale-yellowish siliceous sandstone, of the
red conglomerate, and of the greenish jaspery and pseudo-honestone
varieties. It likewise included three or four widely separated layers
of a brown limestone, abounding with shells immediately to be
described. This pile of strata was in parts traversed by many veins of
gypsum. The calcareous shale-rock, though when freshly broken quite
black, weathers into an ash- colour: in which respect and in general
appearance, it perfectly resembles those great fossiliferous beds of
the Peuquenes range, alternating with gypsum and red sandstone,
described in the last chapter.
The shells out of the layers of brown limestone, included in the black
calcareous shale-rock, which latter, as just stated, replaces the white
siliceous sandstone, consist of:—
Pecten Dufreynoyi, d’Orbigny, “Voyage” Part Pal. Turritella Andii,
d’Orbigny, “Voyage” Part Pal.
Astarte Darwinii, E. Forbes. Gryphaea Darwinii, E. Forbes.
An intermediate form between G. gigantea and G. incurva.
Gryphaea nov. spec.?, E. Forbes. Perna Americana, E. Forbes. Avicula,
nov. spec.
Considered by Mr. G.B. Sowerby as the A. echinata, by M. d’Orbigny as
certainly a new and distinct species, having a Jurassic aspect. The
specimen has been unfortunately lost.
Terebratula aenigma, d’Orbigny, (var. of do. E. Forbes.)
This is the same variety, with that from Guasco, considered by M.
D’Orbigny to be a distinct species from his T. aenigma, and related to
T. obsoleta.
Plagiostoma and Ammonites, fragments of.
The lower layers of the limestone contained thousands of the Gryphaea;
and the upper ones as many of the Turritella, with the Gryphaea (nov.
species) and Serpulae adhering to them; in all the layers, the
Terebratula and fragments of the Pecten were included. It was evident,
from the manner in which species were grouped together, that they had
lived where now embedded. Before making any further remarks, I may
state, that higher up this same valley we shall again meet with a
similar association of shells; and in the great Despoblado Valley,
which branches off near the town from that of Copiapo, the Pecten
Dufreynoyi, some Gryphites (I believe G. Darwinii), and the TRUE
Terebratula aenigma of d’Orbigny were found together in an equivalent
formation, as will be hereafter seen. A specimen also, I may add, of
the true T. aenigma, was given me from the neighbourhood of the famous
silver mines of Chanuncillo, a little south of the valley of the
Copiapo, and these mines, from their position, I have no doubt, lie
within the great gypseous formation: the rocks close to one of the
silver veins, judging from fragments shown me, resemble those singular
metamorphosed deposits from the mining district of Arqueros near
Coquimbo.
I will reiterate the evidence on the association of these several
shells in the several localities.
COQUIMBO.
In the same bed, Rio Claro: Pecten Dufreynoyi. Ostrea hemispherica.
Terebratula aenigma. Spirifer linguiferoides.
Same bed, near Arqueros: Hippurites Chilensis. Gryphaea orientalis.
Collected by M. Domeyko from the same locality, apparently near
Arqueros: Terebratula aenigma and Terebratula ignaciana, in same block
of limestone: Pecten Dufreynoyi. Ostrea hemispherica. Hippurites
Chilensis. Turritella Andii. Nautilus Domeykus.
GUASCO.
In a collection from the Cordillera, given me: the specimens all in the
same condition: Pecten Dufreynoyi. Turritella Andii. Terebratula
ignaciana. Terebratula aenigma, var. Spirifer Chilensis.
COPIAPO.
Mingled together in alternating beds in the main valley of Copiapo near
Las Amolanas, and likewise higher up the valley: Pecten Dufreynoyi.
Turritella Andii. Terebratula aenigma, var. as at Guasco. Astarte
Darwinii. Gryphaea Darwinii. Gryphaea nov. species? Perna Americana.
Avicula, nov. species.
Main valley of Copiapo, apparently same formation with that of
Amolanas: Terebratula aenigma (true).
In the same bed, high up the great lateral valley of the Despoblado, in
the ravine of Maricongo: Terebratula aenigma (true). Pecten Dufreynoyi.
Gryphaea Darwinii?
Considering this table, I think it is impossible to doubt that all
these fossils belong to the same formation. If, however, the species
from Las Amolanas, in the Valley of Copiapo, had, as in the case of
those from Guasco, been separately examined, they would probably have
been ranked as oolitic; for, although no Spirifers were found here, all
the other species, with the exception of the Pecten, Turritella, and
Astarte, have a more ancient aspect than cretaceous forms. On the other
hand, taking into account the evidence derived from the cretaceous
character of these three shells, and of the Hippurites, Gryphaea
orientalis, and Ostrea, from Coquimbo, we are driven back to the
provisional name already used of cretaceo-oolitic. From geological
evidence, I believe this formation to be the equivalent of the
Neocomian beds of the Cordillera of Central Chile.
To return to our section near Las Amolanas:—Above the yellow siliceous
sandstone, or the equivalent calcareous slate-rock, with its bands of
fossil-shells, according as the one or other prevails, there is a pile
of strata, which cannot be less than from two to three thousand feet in
thickness, in main part composed of a coarse, bright red conglomerate,
with many intercalated beds of red sandstone, and some of green and
other coloured porcelain-jaspery layers. The included pebbles are
well-rounded, varying from the size of an egg to that of a
cricket-ball, with a few larger; and they consist chiefly of
porphyries. The basis of the conglomerate, as well as some of the
alternating thin beds, are formed of a red, rather harsh, easily
fusible sandstone, with crystalline calcareous particles. This whole
great pile is remarkable from the thousands of huge, embedded,
silicified trunks of trees, one of which was eight feet long, and
another eighteen feet in circumference: how marvellous it is, that
every vessel in so thick a mass of wood should have been converted into
silex! I brought home many specimens, and all of them, according to Mr.
R. Brown, present a coniferous structure.
Above this great conglomerate, we have from two to three hundred feet
in thickness of red sandstone; and above this, a stratum of black
calcareous slate-rock, like that which alternates with and replaces the
underlying yellowish-white, siliceous sandstone. Close to the junction
between this upper black slate-rock and the upper red sandstone, I
found the Gryphaea Darwinii, the Turritella Andii, and vast numbers of
a bivalve, too imperfect to be recognised. Hence we see that, as far as
the evidence of these two shells serves—and the Turritella is an
eminently characteristic species—the whole thickness of this vast pile
of strata belongs to the same age. Again, above the last-mentioned
upper red sandstone, there were several alternations of the black,
calcareous slate-rock; but I was unable to ascend to them. All these
uppermost strata, like the lower ones, vary extremely in character in
short horizontal distances. The gypseous formation, as here seen, has a
coarser, more mechanical texture, and contains much more siliceous
matter than the corresponding beds lower down the valley. Its total
thickness, together with the upper beds of the porphyritic
conglomerate, I estimated at least at 8,000 feet; and only a small
portion of the porphyritic conglomerate, which on the eastern flank of
the fourth axis of elevation appeared to be from fifteen hundred to two
thousand feet thick, is here included. As corroborative of the great
thickness of the gypseous formation, I may mention that in the
Despoblado Valley (which branches from the main valley a little above
the town of Copiapo) I found a corresponding pile of red and white
sandstones, and of dark, calcareous, semi-jaspery mudstones, rising
from a nearly level surface and thrown into an absolutely vertical
position; so that, by pacing, I ascertained their thickness to be
nearly two thousand seven hundred feet; taking this as a standard of
comparison, I estimated the thickness of the strata ABOVE the
porphyritic conglomerate at 7,000 feet.
The fossils before enumerated, from the limestone-layers in the whitish
siliceous sandstone, are now covered, on the least computation, by
strata from 5,000 to 6,000 feet in thickness. Professor E. Forbes
thinks that these shells probably lived at a depth of from about 30 to
40 fathoms, that is from 180 to 240 feet; anyhow, it is impossible that
they could have lived at the depth of from 5,000 to 6,000 feet. Hence
in this case, as in that of the Puente del Inca, we may safely conclude
that the bottom of the sea on which the shells lived, subsided, so as
to receive the superincumbent submarine strata: and this subsidence
must have taken place during the existence of these shells; for, as I
have shown, some of them occur high up as well as low down in the
series. That the bottom of the sea subsided, is in harmony with the
presence of the layers of coarse, well- rounded pebbles included
throughout this whole pile of strata, as well as of the great upper
mass of conglomerate from 2,000 to 3,000 feet thick; for coarse gravel
could hardly have been formed or spread out at the profound depths
indicated by the thickness of the strata. The subsidence, also, must
have been slow to have allowed of this often-recurrent spreading out of
the pebbles. Moreover, we shall presently see that the surfaces of some
of the streams of porphyritic lava beneath the gypseous formation, are
so highly amygdaloidal that it is scarcely possible to believe that
they flowed under the vast pressure of a deep ocean. The conclusion of
a great subsidence during the existence of these cretaceo-oolitic
fossils, may, I believe, be extended to the district of Coquimbo,
although owing to the fossiliferous beds there not being directly
covered by the upper gypseous strata, which in the section north of the
valley are about 6,000 feet in thickness, I did not there insist on
this conclusion.
The pebbles in the above conglomerates, both in the upper and lower
beds, are all well rounded, and, though chiefly composed of various
porphyries, there are some of red sandstone and of a jaspery stone,
both like the rocks intercalated in layers in this same gypseous
formation; there was one pebble of mica-slate and some of quartz,
together with many particles of quartz. In these respects there is a
wide difference between the gypseous conglomerates and those of the
porphyritic-conglomerate formation, in which latter, angular and
rounded fragments, almost exclusively composed of porphyries, are
mingled together, and which, as already often remarked, probably were
ejected from craters deep under the sea. From these facts I conclude,
that during the formation of the conglomerates, land existed in the
neighbourhood, on the shores of which the innumerable pebbles were
rounded and thence dispersed, and on which the coniferous forests
flourished—for it is improbable that so many thousand logs of wood
should have drifted from any great distance. This land, probably
islands, must have been mainly formed of porphyries, with some
mica-slate, whence the quartz was derived, and with some red sandstone
and jaspery rocks. This latter fact is important, as it shows that in
this district, even previously to the deposition of the lower gypseous
or cretaceo-oolitic beds, strata of an analogous nature had elsewhere,
no doubt in the more central ranges of the Cordillera, been elevated;
thus recalling to our minds the relations of the Cumbre and Uspallata
chains. Having already referred to the great lateral valley of the
Despoblado, I may mention that above the 2,700 feet of red and white
sandstone and dark mudstone, there is a vast mass of coarse, hard, red
conglomerate, some thousand feet in thickness, which contains much
silicified wood, and evidently corresponds with the great upper
conglomerate at Las Amolanas: here, however, the conglomerate consists
almost exclusively of pebbles of granite, and of disintegrated crystals
of reddish feldspar and quartz firmly recemented together. In this
case, we may conclude that the land whence the pebbles were derived,
and on which the now silicified trees once flourished, was formed of
granite.
The mountains near Las Amolanas, composed of the cretaceo-oolitic
strata, are interlaced with dikes like a spider’s web, to an extent
which I have never seen equalled, except in the denuded interior of a
volcanic crater: north and south lines, however, predominate. These
dikes are composed of green, white, and blackish rocks, all porphyritic
with feldspar, and often with large crystals of hornblende. The white
varieties approach closely in character to andesite, which composes as
we have seen, the injected axes of so many of the lines of elevation.
Some of the green varieties are finely laminated, parallel to the walls
of the dikes.
SIXTH AXIS OF ELEVATION (VALLEY OF COPIAPO).
This axis consists of a broad mountainous mass [O] of andesite,
composed of albite, brown mica, and chlorite, passing into andesitic
granite, with quartz: on its western side it has thrown off, at a
considerable angle, a thick mass of stratified porphyries, including
much epidote [NN], and remarkable only from being divided into very
thin beds, as highly amygdaloidal on their surfaces as subaerial
lava-streams are often vesicular. This porphyritic formation is
conformably covered, as seen some way up the ravine of Jolquera, by a
mere remnant of the lower part of the cretaceo-oolitic formation [MM],
which in one part encases, as represented in the coloured section, the
foot of the andesitic axis [L], of the already described fifth line,
and in another part entirely conceals it: in this latter case, the
gypseous or cretaceo-oolitic strata falsely appeared to dip under the
porphyritic conglomerate of the fifth axis. The lowest bed of the
gypseous formation, as seen here [M], is of yellowish siliceous
sandstone, precisely like that of Amolanas, interlaced in parts with
veins of gypsum, and including layers of the black, calcareous,
non-fissile slate-rock: the Turritella Andii, Pecten Dufreynoyi,
Terebratula aenigma, var., and some Gryphites were embedded in these
layers. The sandstone varies in thickness from only twenty to eighty
feet; and this variation is caused by the inequalities in the upper
surface of an underlying stream of purple claystone porphyry. Hence the
above fossils here lie at the very base of the gypseous or
cretaceo-oolitic formation, and hence they were probably once covered
up by strata about seven thousand feet in thickness: it is, however,
possible, though from the nature of all the other sections in this
district not probable, that the porphyritic claystone lava may in this
case have invaded a higher level in the series. Above the sandstone
there is a considerable mass of much indurated, purplish-black,
calcareous claystone, allied in nature to the often-mentioned black
calcareous slate- rock. Eastward of the broad andesitic axis of this
sixth line, and penetrated by many dikes from it, there is a great
formation [P] of mica-schist, with its usual variations, and passing in
one part into a ferruginous quartz-rock. The folia are curved and
highly inclined, generally dipping eastward. It is probable that this
mica-schist is an old formation, connected with the granitic rocks and
metamorphic schists near the coast; and that the one fragment of
mica-slate, and the pebbles of quartz low down in the gypseous
formation at Las Amolanas, have been derived from it. The mica-schist
is succeeded by stratified porphyritic conglomerate [Q] of great
thickness, dipping eastward with a high inclination: I have included
this latter mountain-mass in the same anticlinal axis with the
porphyritic streams [NN]; but I am far from sure that the two masses
may not have been independently upheaved.
SEVENTH AXIS OF ELEVATION.
Proceeding up the ravine, we come to another mass [R] of andesite; and
beyond this, we again have a very thick, stratified porphyritic
formation [S], dipping at a small angle eastward, and forming the basal
part of the main Cordillera. I did not ascend the ravine any higher;
but here, near Castano, I examined several sections, of which I will
not give the details, only observing, that the porphyritic beds, or
submarine lavas, preponderate greatly in bulk over the alternating
sedimentary layers, which have been but little metamorphosed: these
latter consist of fine-grained red tuffs and of whitish volcanic
grit-stones, together with much of a singular, compact rock, having an
almost crystalline basis, finely brecciated with red and green
fragments, and occasionally including a few large pebbles. The
porphyritic lavas are highly amygdaloidal, both on their upper and
lower surfaces; they consist chiefly of claystone porphyry, but with
one common variety, like some of the streams at the Puente del Inca,
having a grey mottled basis, abounding with crystals of red hydrous
oxide of iron, green ones apparently of epidote, and a few glassy ones
of feldspar. This pile of strata differs considerably from the basal
strata of the Cordillera in Central Chile, and may possibly belong to
the upper and gypseous series: I saw, however, in the bed of the
valley, one fragment of porphyritic breccia-conglomerate, exactly like
those great masses met with in the more southern parts of Chile.
Finally, I must observe, that though I have described between the town
of Copiapo and the western flank of the main Cordillera seven or eight
axes of elevation, extending nearly north and south, it must not be
supposed that they all run continuously for great distances. As was
stated to be the case in our sections across the Cordillera of Central
Chile, so here most of the lines of elevation, with the exception of
the first, third, and fifth, are very short. The stratification is
everywhere disturbed and intricate; nowhere have I seen more numerous
faults and dikes. The whole district, from the sea to the Cordillera,
is more or less metalliferous; and I heard of gold, silver, copper,
lead, mercury, and iron veins. The metamorphic action, even in the
lower strata, has certainly been far less here than in Central Chile.
VALLEY OF THE DESPOBLADO.
This great barren valley, which has already been alluded to, enters the
main valley of Copiapo a little above the town: it runs at first
northerly, then N.E., and more easterly into the Cordillera; I followed
its dreary course to the foot of the first main ridge. I will not give
a detailed section, because it would be essentially similar to that
already given, and because the stratification is exceedingly
complicated. After leaving the plutonic hills near the town, I met
first, as in the main valley, with the gypseous formation, having the
same diversified character as before, and soon afterwards with masses
of porphyritic conglomerate, about one thousand feet in thickness. In
the lower part of this formation there were very thick beds composed of
fragments of claystone porphyries, both angular and rounded, with the
smaller ones partially blended together and the basis rendered
porphyritic; these beds separated distinct streams, from sixty to
eighty feet in thickness, of claystone lavas. Near Paipote, also, there
was much true porphyritic breccia-conglomerate: nevertheless, few of
these masses were metamorphosed to the same degree with the
corresponding formation in Central Chile. I did not meet in this valley
with any true andesite, but only with imperfect andesitic porphyry,
including large crystals of hornblende: numerous as have been the
varieties of intrusive porphyries already mentioned, there were here
mountains composed of a new kind, having a compact, smooth,
cream-coloured basis, including only a few crystals of feldspar, and
mottled with dendritic spots of oxide of iron. There were also some
mountains of a porphyry with a brick-red basis, containing irregular,
often lens-shaped, patches of compact feldspar, and crystals of
feldspar, which latter to my surprise I find to be orthite.
At the foot of the first ridge of the main Cordillera, in the ravine of
Maricongo, and at an elevation which, from the extreme coldness and
appearance of the vegetation, I estimated at about ten thousand feet, I
found beds of white sandstone and of limestone including the Pecten
Dufreynoyi, Terebratula aenigma, and some Gryphites. This ridge throws
the water on the one hand into the Pacific, and on the other, as I was
informed, into a great gravel-covered, basin-like plain, including a
salt- lake, and without any drainage-exit. In crossing the Cordillera
by this Pass, it is said that three principal ridges must be traversed,
instead of two, or only one as in Central Chile.
The crest of this first main ridge and the surrounding mountains, with
the exception of a few lofty pinnacles, are capped by a great thickness
of a horizontally stratified, tufaceous deposit. The lowest bed is of a
pale purple colour, hard, fine-grained, and full of broken crystals of
feldspar and scales of mica. The middle bed is coarser, and less hard,
and hence weathers into very sharp pinnacles; it includes very small
fragments of granite, and innumerable ones of all sizes of grey
vesicular trachyte, some of which were distinctly rounded. The
uppermost bed is about two hundred feet in thickness, of a darker
colour and apparently hard: but I had not time to ascend to it. These
three horizontal beds may be seen for the distance of many leagues,
especially westward or in the direction of the Pacific, capping the
summits of the mountains, and standing on the opposite sides of the
immense valleys at exactly corresponding heights. If united they would
form a plain, inclined very slightly towards the Pacific; the beds
become thinner in this direction, and the tuff (judging from one point
to which I ascended, some way down the valley) finer-grained and of
less specific gravity, though still compact and sonorous under the
hammer. The gently inclined, almost horizontal stratification, the
presence of some rounded pebbles, and the compactness of the lowest
bed, though rendering it probable, would not have convinced me that
this mass had been of subaqueous origin, for it is known that volcanic
ashes falling on land and moistened by rain often become hard and
stratified; but beds thus originating, and owing their consolidation to
atmospheric moisture, would have covered almost equally every
neighbouring summit, high and low, and would not have left those above
a certain exact level absolutely bare; this circumstance seems to me to
prove that the volcanic ejections were arrested at their present,
widely extended, equable level, and there consolidated by some other
means than simple atmospheric moisture; and this no doubt must have
been a sheet of water. A lake at this great height, and without a
barrier on any one side, is out of the question; consequently we must
conclude that the tufaceous matter was anciently deposited beneath the
sea. It was certainly deposited before the excavation of the valleys,
or at least before their final enlargement (I have endeavoured to show
in my “Journal” etc. (2nd edition) page 355, that this arid valley was
left by the retreating sea, as the land slowly rose, in the state in
which we now see it.); and I may add, that Mr. Lambert, a gentleman
well acquainted with this country, informs me, that in ascending the
ravine of Santandres (which branches off from the Despoblado) he met
with streams of lava and much erupted matter capping all the hills of
granite and porphyry, with the exception of some projecting points; he
also remarked that the valleys had been excavated subsequently to these
eruptions.
This volcanic formation, which I am informed by Mr. Lambert extends far
northward, is of interest, as typifying what has taken place on a
grander scale on the corresponding western side of the Cordillera of
Peru. Under another point of view, however, it possesses a far higher
interest, as confirming that conclusion drawn from the structure of the
fringes of stratified shingle which are prolonged from the plains at
the foot of the Cordillera far up the valleys,—namely, that this great
range has been elevated in mass to a height of between eight and nine
thousand feet (I may here mention that on the south side of the main
valley of Copiapo, near Potrero Seco, the mountains are capped by a
thick mass of horizontally stratified shingle, at a height which I
estimated at between fifteen hundred and two thousand feet above the
bed of the valley. This shingle, I believe, forms the edge of a wide
plain, which stretches southwards between two mountain ranges.); and
now, judging from this tufaceous deposit, we may conclude that the
horizontal elevation has been in the district of Copiapo about ten
thousand feet.
(FIGURE 24.)
In the valley of the Despoblado, the stratification, as before remarked
has been much disturbed, and in some points to a greater degree than I
have anywhere else seen. I will give two cases: a very thick mass of
thinly stratified red sandstone, including beds of conglomerate, has
been crushed together (as represented in Figure 24) into a yoke or
urn-formed trough, so that the strata on both sides have been folded
inwards: on the right hand the properly underlying porphyritic
claystone conglomerate is seen overlying the sandstone, but it soon
becomes vertical, and then is inclined towards the trough, so that the
beds radiate like the spokes of a wheel: on the left hand, the inverted
porphyritic conglomerate also assumes a dip towards the trough, not
gradually, as on the right hand, but by means of a vertical fault and
synclinal break; and a little still further on towards the left, there
is a second great oblique fault (both shown by the arrow- lines), with
the strata dipping to a directly opposite point; these mountains are
intersected by infinitely numerous dikes, some of which can be seen to
rise from hummocks of greenstone, and can be traced for thousands of
feet. In the second case, two low ridges trend together and unite at
the head of a little wedge-shaped valley: throughout the right- hand
ridge, the strata dip at 45 degrees to the east; in the left-hand
ridge, we have the very same strata and at first with exactly the same
dip; but in following this ridge up the valley, the strata are seen
very regularly to become more and more inclined until they stand
vertical, they then gradually fall over (the basset edges forming
symmetrical serpentine lines along the crest), till at the very head of
the valley they are reversed at an angle of 45 degrees: so that at this
point the beds have been turned through an angle of 135 degrees; and
here there is a kind of anticlinal axis, with the strata on both sides
dipping to opposite points at an angle of 45 degrees, but those on the
left hand upside down.
ON THE ERUPTIVE SOURCES OF THE PORPHYRITIC CLAYSTONE AND GREENSTONE
LAVAS.
In Central Chile, from the extreme metamorphic action, it is in most
parts difficult to distinguish between the streams of porphyritic lava
and the porphyritic breccia-conglomerate, but here, at Copiapo, they
are generally perfectly distinct, and in the Despoblado, I saw for the
first time, two great strata of purple claystone porphyry, after having
been for a considerable space closely united together, one above the
other, become separated by a mass of fragmentary matter, and then both
thin out;—the lower one more rapidly than the upper and greater stream.
Considering the number and thickness of the streams of porphyritic
lava, and the great thickness of the beds of breccia-conglomerate,
there can be little doubt that the sources of eruption must originally
have been numerous: nevertheless, it is now most difficult even to
conjecture the precise point of any one of the ancient submarine
craters. I have repeatedly observed mountains of porphyries, more or
less distinctly stratified towards their summits or on their flanks,
without a trace of stratification in their central and basal parts: in
most cases, I believe this is simply due either to the obliterating
effects of metamorphic action, or to such parts having been mainly
formed of intrusive porphyries, or to both causes conjoined; in some
instances, however, it appeared to me very probable that the great
central unstratified masses of porphyry were the now partially denuded
nuclei of the old submarine volcanoes, and that the stratified parts
marked the points whence the streams flowed. In one case alone, and it
was in this Valley of the Despoblado, I was able actually to trace a
thick stratum of purplish porphyry, which for a space of some miles
conformably overlay the usual alternating beds of breccia-conglomerates
and claystone lavas, until it became united with, and blended into, a
mountainous mass of various unstratified porphyries.
The difficulty of tracing the streams of porphyries to their ancient
and doubtless numerous eruptive sources, may be partly explained by the
very general disturbance which the Cordillera in most parts has
suffered; but I strongly suspect that there is a more specific cause,
namely, THAT THE ORIGINAL POINTS OF ERUPTION TEND TO BECOME THE POINTS
OF INJECTION. This in itself does not seem improbable; for where the
earth’s crust has once yielded, it would be liable to yield again,
though the liquified intrusive matter might not be any longer enabled
to reach the submarine surface and flow as lava. I have been led to
this conclusion, from having so frequently observed that, where part of
an unstratified mountain-mass resembled in mineralogical character the
adjoining streams or strata, there were several other kinds of
intrusive porphyries and andesitic rocks injected into the same point.
As these intrusive mountain-masses form most of the axes-lines in the
Cordillera, whether anticlinal, uniclinal, or synclinal, and as the
main valleys have generally been hollowed out along these lines, the
intrusive masses have generally suffered much denudation. Hence they
are apt to stand in some degree isolated, and to be situated at the
points where the valleys abruptly bend, or where the main tributaries
enter. On this view of there being a tendency in the old points of
eruption to become the points of subsequent injection and disturbance,
and consequently of denudation, it ceases to be surprising that the
streams of lava in the porphyritic claystone conglomerate formation,
and in other analogous cases, should most rarely be traceable to their
actual sources.
IQUIQUE, SOUTHERN PERU.
Differently from what we have seen throughout Chile, the coast here is
formed not by the granitic series, but by an escarpment of the
porphyritic conglomerate formation, between two and three thousand feet
in height. (The lowest point, where the road crosses the
coast-escarpment, is 1,900 feet by the barometer above the level of the
sea.) I had time only for a very short examination; the chief part of
the escarpment appears to be composed of various reddish and purple,
sometimes laminated, porphyries, resembling those of Chile; and I saw
some of the porphyritic breccia-conglomerate; the stratification
appeared but little inclined. The uppermost part, judging from the
rocks near the famous silver mine of Huantajaya, consists of laminated,
impure, argillaceous, purplish-grey limestone, associated, I believe,
with some purple sandstone. (Mr. Bollaert has described “Geological
Proceedings” volume 2 page 598, a singular mass of stratified detritus,
gravel, and sand, eighty-one yards in thickness, overlying the
limestone, and abounding with loose masses of silver ore. The miners
believe that they can attribute these masses to their proper veins.) In
the limestone shells are found: the three following species were given
me:—
Lucina Americana, E. Forbes. Terebratula inca, E. Forbes. Terebratula
aenigma, D’Orbigny.
This latter species we have seen associated with the fossils of which
lists have been given in this chapter, in two places in the valley of
Coquimbo, and in the ravine of Maricongo at Copiapo. Considering this
fact, and the superposition of these beds on the porphyritic
conglomerate formation; and, as we shall immediately see, from their
containing much gypsum, and from their otherwise close general
resemblance in mineralogical nature with the strata described in the
valley of Copiapo, I have little doubt that these fossiliferous beds of
Iquique belong to the great cretaceo-oolitic formation of Northern
Chile. Iquique is situated seven degrees latitude north of Copiapo; and
I may here mention, that an Ammonites, nov. species, and an Astarte,
nov. species, were given me from the Cerro Pasco, about ten degrees of
latitude north of Iquique, and M. D’Orbigny thinks that they probably
indicate a Neocomian formation. Again, fifteen degrees of latitude
northward, in Colombia, there is a grand fossiliferous deposit, now
well known from the labours of Von Buch, Lea, d’Orbigny, and Forbes,
which belongs to the earlier stages of the cretaceous system. Hence,
bearing in mind the character of the few fossils from Tierra del Fuego,
there is some evidence that a great portion of the stratified deposits
of the whole vast range of the South American Cordillera belongs to
about the same geological epoch.
Proceeding from the coast escarpment inwards, I crossed, in a space of
about thirty miles, an elevated undulatory district, with the beds
dipping in various directions. The rocks are of many kinds,—white
laminated, sometimes siliceous sandstone,—purple and red sandstone,
sometimes so highly calcareous as to have a crystalline
fracture,—argillaceous limestone,—black calcareous slate-rock, like
that so often described at Copiapo and other places,—thinly laminated,
fine-grained, greenish, indurated, sedimentary, fusible rocks,
approaching in character to the so- called pseudo-honestone of Chile,
including thin contemporaneous veins of gypsum,—and lastly, much
calcareous, laminated porcelain jasper, of a green colour, with red
spots, and of extremely easy fusibility: I noticed one conformable
stratum of a freckled-brown, feldspathic lava. I may here mention that
I heard of great beds of gypsum in the Cordillera. The only novel point
in this formation, is the presence of innumerable thin layers of
rock-salt, alternating with the laminated and hard, but sometimes
earthy, yellowish, or bright red and ferruginous sandstones. The
thickest layer of salt was only two inches, and it thinned out at both
ends. On one of these saliferous masses I noticed a stratum about
twelve feet thick, of dark-brown, hard brecciated, easily fusible rock,
containing grains of quartz and of black oxide of iron, together with
numerous imperfect fragments of shells. The problem of the origin of
salt is so obscure, that every fact, even geographical position, is
worth recording. (It is well known that stratified salt is found in
several places on the shores of Peru. The island of San Lorenzo, off
Lima, is composed of a pile of thin strata, about eight hundred feet in
thickness, composed of yellowish and purplish, hard siliceous, or
earthy sandstones, alternating with thin layers of shale, which in
places passes into a greenish, semi-porcellanic, fusible rock. There
are some thin beds of reddish mudstone, and soft ferruginous
rotten-stones, with layers of gypsum. In nearly all these varieties,
especially in the softer sandstones, there are numerous thin seams of
rock-salt: I was informed that one layer has been found two inches in
thickness. The manner in which the minutest fissures of the dislocated
beds have been penetrated by the salt, apparently by subsequent
infiltration, is very curious. On the south side of the island, layers
of coal and of impure limestone have been discovered. Hence we here
have salt, gypsum, and coal associated together. The strata include
veins of quartz, carbonate of lime, and iron pyrites; they have been
dislocated by an injected mass of greenish-brown feldspathic trap. Not
only is salt abundant on the extreme western limits of the district
between the Cordillera and the Pacific, but, according to Helms, it is
found in the outlying low hills on the eastern flank of the Cordillera.
These facts appear to me opposed to the theory, that rock-salt is due
to the sinking of water, charged with salt, in mediterranean spaces of
the ocean. The general character of the geology of these countries
would rather lead to the opinion, that its origin is in some way
connected with volcanic heat at the bottom of the sea: see on this
subject Sir R. Murchison “Anniversary Address to the Geological
Society” 1843 page 65.) With the exception of these saliferous beds,
most of the rocks as already remarked, present a striking general
resemblance with the upper parts of the gypseous or cretaceo-oolitic
formation of Chile.
METALLIFEROUS VEINS.
I have only a few remarks to make on this subject: in nine mining
districts, some of them of considerable extent, which I visited in
CENTRAL Chile, I found the PRINCIPAL veins running from between [N. and
N.W.] to [S. and S.E.] (These mining districts are Yaquil near
Nancagua, where the direction of the chief veins, to which only in all
cases I refer, is north and south; in the Uspallata range, the
prevailing line is N.N.W. and S.S.E.; in the C. de Prado, it is N.N.W.
and S.S.E.; near Illapel, it is N. by W. and S. by E.; at Los Hornos
the direction varies from between [N. and N.W.] to [S. and S.E.]; at
the C. de los Hornos (further northward), it is N.N.W. and S.S.E.; at
Panuncillo, it is N.N.W. and S.S.E.; and, lastly, at Arqueros, the
direction is N.W. and S.E.): in some other places, however, their
courses appeared quite irregular, as is said to be generally the case
in the whole valley of Copiapo: at Tambillos, south of Coquimbo, I saw
one large copper vein extending east and west. It is worthy of notice,
that the foliation of the gneiss and mica-slate, where such rocks
occur, certainly tend to run like the metalliferous veins, though often
irregularly, in a direction a little westward of north. At Yaquil, I
observed that the principal auriferous veins ran nearly parallel to the
grain or imperfect cleavage of the surrounding GRANITIC rocks. With
respect to the distribution of the different metals, copper, gold, and
iron are generally associated together, and are most frequently found
(but with many exceptions, as we shall presently see) in the rocks of
the lower series, between the Cordillera and the Pacific, namely, in
granite, syenite, altered feldspathic clay-slate, gneiss, and as near
Guasco mica-schist. The copper-ores consist of sulphurets, oxides, and
carbonates, sometimes with laminae of native metal: I was assured that
in some cases (as at Panuncillo S.E. of Coquimbo), the upper part of
the same vein contains oxides, and the lower part sulphurets of copper.
(The same fact has been observed by Mr. Taylor in Cuba: “London
Philosophical Journal” volume 11 page 21.) Gold occurs in its native
form; it is believed that, in many cases, the upper part of the vein is
the most productive part: this fact probably is connected with the
abundance of this metal in the stratified detritus of Chile, which must
have been chiefly derived from the degradation of the upper portions of
the rocks. These superficial beds of well-rounded gravel and sand,
containing gold, appeared to me to have been formed under the sea close
to the beach, during the slow elevation of the land: Schmidtmeyer
remarks that in Chile gold is sought for in shelving banks at the
height of some feet on the sides of the streams, and not in their beds,
as would have been the case had this metal been deposited by common
alluvial action. (“Travels in Chile” page 29.) Very frequently the
copper-ores, including some gold, are associated with abundant
micaceous specular iron. Gold is often found in iron-pyrites: at two
gold mines at Yaquil (near Nancagua), I was informed by the proprietor
that in one the gold was always associated with copper-pyrites, and in
the other with iron-pyrites: in this latter case, it is said that if
the vein ceases to contain iron-pyrites, it is yet worth while to
continue the search, but if the iron-pyrites, when it reappears, is not
auriferous, it is better at once to give up working the vein. Although
I believe copper and gold are most frequently found in the lower
granitic and metamorphic schistose series, yet these metals occur both
in the porphyritic conglomerate formation (as on the flanks of the Bell
of Quillota and at Jajuel), and in the superincumbent strata. At Jajuel
I was informed that the copper-ore, with some gold, is found only in
the greenstones and altered feldspathic clay-slate, which alternate
with the purple porphyritic conglomerate. Several gold veins and some
of copper- ore are worked in several parts of the Uspallata range, both
in the metamorphosed strata, which have been shown to have been of
probably subsequent origin to the Neocomian or gypseous formation of
the main Cordillera, and in the intrusive andesitic rocks of that
range. At Los Hornos (N.E. of Illapel), likewise, there are numerous
veins of copper- pyrites and of gold, both in the strata of the
gypseous formation and in the injected hills of andesite and various
porphyries.
Silver, in the form of a chloride, sulphuret, or an amalgam, or in its
native state, and associated with lead and other metals, and at
Arqueros with pure native copper, occurs chiefly in the upper great
gypseous or cretaceo-oolitic formation which forms probably the richest
mass in Chile. We may instance the mining districts of Arqueros near
Coquimbo, and of nearly the whole valley of Copiapo, and of Iquique
(where the principal veins run N.E. by E. and S.W. by W.), in Peru.
Hence comes Molina’s remark, that silver is born in the cold and
solitary deserts of the Upper Cordillera. There are, however,
exceptions to this rule: at Paral (S.E. of Coquimbo) silver is found in
the porphyritic conglomerate formation; as I suspect is likewise the
case at S. Pedro de Nolasko in the Peuquenes Pass. Rich argentiferous
lead is found in the clay-slate of the Uspallata range; and I saw an
old silver-mine in a hill of syenite at the foot of the Bell of
Quillota: I was also assured that silver has been found in the
andesitic and porphyritic region between the town of Copiapo and the
Pacific. I have stated in a previous part of this chapter, that in two
neighbouring mines at Arqueros the veins in one were productive when
they traversed the singular green sedimentary beds, and unproductive
when crossing the reddish beds; whereas at the other mine exactly the
reverse takes place; I have also described the singular and rare case
of numerous particles of native silver and of the chloride being
disseminated in the green rock at the distance of a yard from the vein.
Mercury occurs with silver both at Arqueros and at Copiapo: at the base
of C. de los Hornos (S.E. of Coquimbo, a different place from Los
Hornos, before mentioned) I saw in a syenitic rock numerous quartzose
veins, containing a little cinnabar in nests: there were here other
parallel veins of copper and of a ferrugino-auriferous ore. I believe
tin has never been found in Chile.
From information given me by Mr. Nixon of Yaquil (At the Durazno mine,
the gold is associated with copper-pyrites, and the veins contain large
prisms of plumbago. Crystallised carbonate of lime is one of the
commonest minerals in the matrix of the Chilean veins.), and by others,
it appears that in Chile those veins are generally most permanently
productive, which, consisting of various minerals (sometimes differing
but slightly from the surrounding rocks), include parallel strings RICH
in metals; such a vein is called a veta real. More commonly the mines
are worked only where one, two, or more thin veins or strings running
in a different direction, intersect a POOR “veta real:” it is
unanimously believed that at such points of intersection (cruceros),
the quantity of metal is much greater than that contained in other
parts of the intersecting veins. In some cruceros or points of
intersection, the metals extend even beyond the walls of the main,
broad, stony vein. It is said that the greater the angle of
intersection, the greater the produce; and that nearly parallel strings
attract each other; in the Uspallata range, I observed that numerous
thin auri-ferruginous veins repeatedly ran into knots, and then
branched out again. I have already described the remarkable manner in
which rocks of the Uspallata range are indurated and blackened (as if
by a blast of gunpowder) to a considerable distance from the metallic
veins.
Finally, I may observe, that the presence of metallic veins seems
obviously connected with the presence of intrusive rocks, and with the
degree of metamorphic action which the different districts of Chile
have undergone. (Sir R. Murchison and his fellow travellers have given
some striking facts on this subject in their account of the Ural
Mountains (“Geological Proceedings” volume 3 page 748.) Such
metamorphosed areas are generally accompanied by numerous dikes and
injected masses of andesite and various porphyries: I have in several
places traced the metalliferous veins from the intrusive masses into
the encasing strata. Knowing that the porphyritic conglomerate
formation consists of alternate streams of submarine lavas and of the
debris of anciently erupted rocks, and that the strata of the upper
gypseous formation sometimes include submarine lavas, and are composed
of tuffs, mudstones, and mineral substances, probably due to volcanic
exhalations,—the richness of these strata is highly remarkable when
compared with the erupted beds, often of submarine origin, but NOT
METAMORPHOSED, which compose the numerous islands in the Pacific,
Indian, and Atlantic Oceans; for in these islands metals are entirely
absent, and their nature even unknown to the aborigines.
A SUMMARY OF THE GEOLOGICAL HISTORY OF THE CHILEAN CORDILLERA, AND OF
THE SOUTHERN PARTS OF SOUTH AMERICA.
We have seen that the shores of the Pacific, for a space of 1,200 miles
from Tres Montes to Copiapo, and I believe for a very much greater
distance, are composed, with the exception of the tertiary basins, of
metamorphic schists, plutonic rocks, and more or less altered
clay-slate. On the floor of the ocean thus constituted, vast streams of
various purplish claystone and greenstone porphyries were poured forth,
together with great alternating piles of angular and rounded fragments
of similar rocks ejected from the submarine craters. From the
compactness of the streams and fragments, it is probable that, with the
exception of some districts in Northern Chile, the eruptions took place
in profoundly deep water. The orifices of eruption appear to have been
studded over a breadth, with some outliers, of from fifty to one
hundred miles: and closely enough together, both north and south, and
east and west, for the ejected matter to form a continuous mass, which
in Central Chile is more than a mile in thickness. I traced this
mould-like mass, for only 450 miles; but judging from what I saw at
Iquique, from specimens, and from published accounts, it appears to
have a manifold greater length. In the basal parts of the series, and
especially towards the flanks of the range, mud, since converted into a
feldspathic slaty rock, and sometimes into greenstone, was occasionally
deposited between the beds of erupted matter: with this exception the
uniformity of the porphyritic rocks is very remarkable.
At the period when the claystone and greenstone porphyries nearly or
quite ceased being erupted, that great pile of strata which, from often
abounding with gypsum, I have generally called the gypseous formation
was deposited, and feldspathic lavas, together with other singular
volcanic rocks, were occasionally poured forth: I am far from
pretending that any distinct line of demarcation can be drawn between
this formation and the underlying porphyries and porphyritic
conglomerate, but in a mass of such great thickness, and between beds
of such widely different mineralogical nature, some division was
necessary. At about the commencement of the gypseous period, the bottom
of the sea here seems first to have been peopled by shells, not many in
kind, but abounding in individuals. At the P. del Inca the fossils are
embedded near the base of the formation; in the Peuquenes range, at
different levels, halfway up, and even higher in the series; hence, in
these sections, the whole pile of strata belongs to the same period:
the same remark is applicable to the beds at Copiapo, which attain a
thickness of between seven and eight thousand feet. The fossil shells
in the Cordillera of Central Chile, in the opinion of all the
palaeontologists who have examined them, belong to the earlier stages
of the cretaceous system; whilst in Northern Chile there is a most
singular mixture of cretaceous and oolitic forms: from the geological
relations, however, of these two districts, I cannot but think that
they all belong to nearly the same epoch, which I have provisionally
called cretaceo-oolitic.
The strata in this formation, composed of black calcareous shaly-rocks
of red and white, and sometimes siliceous sandstone, of coarse
conglomerates, limestones, tuffs, dark mudstones, and those singular
fine-grained rocks which I have called pseudo-honestones, vast beds of
gypsum, and many other jaspery and scarcely describable varieties, vary
and replace each other in short horizontal distances, to an extent, I
believe, unequalled even in any tertiary basin. Most of these
substances are easily fusible, and have apparently been derived either
from volcanoes still in quiet action, or from the attrition of volcanic
products. If we picture to ourselves the bottom of the sea, rendered
uneven in an extreme degree, with numerous craters, some few
occasionally in eruption, but the greater number in the state of
solfataras, discharging calcareous, siliceous, ferruginous matters, and
gypsum or sulphuric acid to an amount surpassing, perhaps, even the
existing sulphureous volcanoes of Java (Von Buch’s “Description
Physique des Iles Canaries” page 428.), we shall probably understand
the circumstances under which this singular pile of varying strata was
accumulated. The shells appear to have lived at the quiescent periods
when only limestone or calcareo-argillaceous matter was depositing.
From Dr. Gillies’ account, this gypseous or cretaceo-oolitic formation
extends as far south as the Pass of Planchon, and I followed it
northward at intervals for 500 miles: judging from the character of the
beds with the Terebratula aenigma, at Iquique, it extends from four to
five hundred miles further: and perhaps even for ten degrees of
latitude north of Iquique to the Cerro Pasco, not far from Lima: again,
we know that a cretaceous formation, abounding with fossils, is largely
developed north of the equator, in Colombia: in Tierra del Fuego, at
about this same period, a wide district of clay-slate was deposited,
which in its mineralogical characters and external features, might be
compared to the Silurian regions of North Wales. The gypseous
formation, like that of the porphyritic breccia- conglomerate on which
it rests, is of inconsiderable breadth; though of greater breadth in
Northern than in Central Chile.
As the fossil shells in this formation are covered, in the Peuquenes
ridge, by a great thickness of strata; at the Puente del Inca, by at
least five thousand feet; at Coquimbo, though the superposition there
is less plainly seen, by about six thousand feet; and at Copiapo,
certainly by five or six thousand, and probably by seven thousand feet
(the same species there recurring in the upper and lower parts of the
series), we may feel confident that the bottom of the sea subsided
during this cretaceo-oolitic period, so as to allow of the accumulation
of the superincumbent submarine strata. This conclusion is confirmed
by, or perhaps rather explains, the presence of the many beds at many
levels of coarse conglomerate, the well- rounded pebbles in which we
cannot believe were transported in very deep water. Even the underlying
porphyries at Copiapo. with their highly amygdaloidal surfaces, do not
appear to have flowed under great pressure. The great sinking movement
thus plainly indicated, must have extended in a north and south line
for at least four hundred miles, and probably was co- extensive with
the gypseous formation.
The beds of conglomerate just referred to, and the extraordinarily
numerous silicified trunks of fir-trees at Los Hornos, perhaps at
Coquimbo and at two distant points in the valley of Copiapo, indicate
that land existed at this period in the neighbourhood. This land, or
islands, in the northern part of the district of Copiapo, must have
been almost exclusively composed, judging from the nature of the
pebbles of granite: in the southern parts of Copiapo, it must have been
mainly formed of claystone porphyries, with some mica-schist, and with
much sandstone and jaspery rocks exactly like the rocks in the gypseous
formation, and no doubt belonging to its basal series. In several other
places also, during the accumulation of the gypseous formation, its
basal parts and the underlying porphyritic conglomerate must likewise
have been already partially upheaved and exposed to wear and tear; near
the Puente del Inca and at Coquimbo, there must have existed masses of
mica-schist or some such rock, whence were derived the many small
pebbles of opaque quartz. It follows from these facts, that in some
parts of the Cordillera the upper beds of the gypseous formation must
lie unconformably on the lower beds; and the whole gypseous formation,
in parts, unconformably on the porphyritic conglomerate; although I saw
no such cases, yet in many places the gypseous formation is entirely
absent; and this, although no doubt generally caused by quite
subsequent denudation, may in others be due to the underlying
porphyritic conglomerate having been locally upheaved before the
deposition of the gypseous strata, and thus having become the source of
the pebbles of porphyry embedded in them. In the porphyritic
conglomerate formation, in its lower and middle parts, there is very
rarely any evidence, with the exception of the small quartz pebbles at
Jajuel near Aconcagua, and of the single pebble of granite at Copiapo,
of the existence of neighbouring land: in the upper parts, however, and
especially in the district of Copiapo, the number of thoroughly
well-rounded pebbles of compact porphyries make me believe, that, as
during the prolonged accumulation of the gypseous formation the lower
beds had already been locally upheaved and exposed to wear and tear, so
it was with the porphyritic conglomerate. Hence in following thus far
the geological history of the Cordillera, it may be inferred that the
bed of a deep and open, or nearly open, ocean was filled up by
porphyritic eruptions, aided probably by some general and some local
elevations, to that comparatively shallow level at which the cretaceo-
oolitic shells first lived. At this period, the submarine craters
yielded at intervals a prodigious supply of gypsum and other mineral
exhalations, and occasionally, in certain places poured forth lavas,
chiefly of a feldspathic nature: at this period, islands clothed with
fir-trees and composed of porphyries, primary rocks, and the lower
gypseous strata had already been locally upheaved, and exposed to the
action of the waves;—the general movement, however, at this time having
been over a very wide area, one of slow subsidence, prolonged till the
bed of the sea sank several thousand feet.
In Central Chile, after the deposition of a great thickness of the
gypseous strata, and after their upheaval, by which the Cumbre and
adjoining ranges were formed, a vast pile of tufaceous matter and
submarine lava was accumulated, where the Uspallata chain now stands;
also after the deposition and upheaval of the equivalent gypseous
strata of the Peuquenes range, the great thick mass of conglomerate in
the valley of Tenuyan was accumulated: during the deposition of the
Uspallata strata, we know absolutely, from the buried vertical trees,
that there was a subsidence of some thousand feet; and we may infer
from the nature of the conglomerate in the valley of Tenuyan, that a
similar and perhaps contemporaneous movement there took place. We have,
then, evidence of a second great period of subsidence; and, as in the
case of the subsidence which accompanied the accumulation of the
cretaceo-oolitic strata, so this latter subsidence appears to have been
complicated by alternate or local elevatory movement— for the vertical
trees, buried in the midst of the Uspallata strata, must have grown on
dry land, formed by the upheaval of the lower submarine beds. Presently
I shall have to recapitulate the facts, showing that at a still later
period, namely, at nearly the commencement of the old tertiary deposits
of Patagonia and of Chile, the continent stood at nearly its present
level, and then, for the third time, slowly subsided to the amount of
several hundred feet, and was afterwards slowly re-uplifted to its
present level.
The highest peaks of the Cordillera appear to consist of active or more
commonly dormant volcanoes,—such as Tupungato, Maypu, and Aconcagua,
which latter stands 23,000 feet above the level of the sea, and many
others. The next highest peaks are formed of the gypseous and
porphyritic strata, thrown into vertical or highly inclined positions.
Besides the elevation thus gained by angular displacements, I infer,
without any hesitation—from the stratified gravel-fringes which gently
slope up the valleys of the Cordillera from the gravel-capped plains at
their base, which latter are connected with the plains, still covered
with recent shells on the coast— that this great range has been
upheaved in mass by a slow movement, to an amount of at least 8,000
feet. In the Despoblado Valley, north of Copiapo, the horizontal
elevation, judging from the compact, stratified tufaceous deposit,
capping the distant mountains at corresponding heights, was about ten
thousand feet. It is very possible, or rather probable, that this
elevation in mass may not have been strictly horizontal, but more
energetic under the Cordillera, than towards the coast on either side;
nevertheless, movements of this kind may be conveniently distinguished
from those by which strata have been abruptly broken and upturned. When
viewing the Cordillera, before having read Mr. Hopkins’s profound
“Researches on Physical Geology,” the conviction was impressed on me,
that the angular dislocations, however violent, were quite subordinate
in importance to the great upward movement in mass, and that they had
been caused by the edges of the wide fissures, which necessarily
resulted from the tension of the elevated area, having yielded to the
inward rush of fluidified rock, and having thus been upturned.
The ridges formed by the angularly upheaved strata are seldom of great
length: in the central parts of the Cordillera they are generally
parallel to each other, and run in north and south lines; but towards
the flanks they often extend more or less obliquely. The angular
displacement has been much more violent in the central than in the
exterior MAIN lines; but it has likewise been violent in some of the
MINOR lines on the extreme flanks. The violence has been very unequal
on the same short lines; the crust having apparently tended to yield on
certain points along the lines of fissures. These points, I have
endeavoured to show, were probably first foci of eruption, and
afterwards of injected masses of porphyry and andesite. (Sir R.
Murchison and his companions state “Geological Proceedings” volume 3
page 747, that no true granite appears in the higher Ural Mountains;
but that syenitic greenstone—a rock closely analogous to our
andesite—is far the most abundant of the intrusive masses.) The close
similarity of the andesitic granites and porphyries, throughout Chile,
Tierra del Fuego, and even in Peru, is very remarkable. The prevalence
of feldspar cleaving like albite, is common not only to the andesites,
but (as I infer from the high authority of Professor G. Rose, as well
as from my own measurements) to the various claystone and greenstone
porphyries, and to the trachytic lavas of the Cordillera. The andesitic
rocks have in most cases been the last injected ones, and they probably
form a continuous dome under this great range: they stand in intimate
relationship with the modern lavas; and they seem to have been the
immediate agent in metamorphosing the porphyritic conglomerate
formation, and often likewise the gypseous strata, to the extraordinary
extent to which they have suffered.
With respect to the age at which the several parallel ridges composing
the Cordillera were upthrown, I have little evidence. Many of them may
have been contemporaneously elevated and injected in the same manner as
in volcanic archipelagoes lavas are contemporaneously ejected on the
parallel lines of fissure. (“Volcanic Islands” etc.) But the pebbles
apparently derived from the wear and tear of the porphyritic
conglomerate formation, which are occasionally present in the upper
parts of this same formation, and are often present in the gypseous
formation, together with the pebbles from the basal parts of the latter
formation in its upper strata, render it almost certain that portions,
we may infer ridges, of these two formations were successively
upheaved. In the case of the gigantic Portillo range, we may feel
almost certain that a preexisting granitic line was upraised (not by a
single blow, as shown by the highly inclined basaltic streams in the
valley on its eastern flank) at a period long subsequent to the
upheavement of the parallel Peuquenes range. (I have endeavoured to
show in my “Journal” 2nd edition page 321, that the singular fact of
the river, which drains the valley between these two ranges, passing
through the Portillo and higher line, is explained by its slow and
subsequent elevation. There are many analogous cases in the drainage of
rivers: see “Edinburgh New Philosophical Journal” volume 28 pages 33
and 44.) Again, subsequently to the upheavement of the Cumbre chain,
that of Uspallata was formed and elevated; and afterwards, I may add,
in the plain of Uspallata, beds of sand and gravel were violently
upthrown. The manner in which the various kinds of porphyries and
andesites have been injected one into the other, and in which the
infinitely numerous dikes of various composition intersect each other,
plainly show that the stratified crust has been stretched and yielded
many times over the same points. With respect to the age of the axes of
elevation between the Pacific and the Cordillera, I know little: but
there are some lines which must—namely, those running north and south
in Chiloe, those eight or nine east and west, parallel, far-extended,
most symmetrical uniclinal lines at P. Rumena, and the short N.W.-S.E.
and N.E.- S.W. lines at Concepcion—have been upheaved long after the
formation of the Cordillera. Even during the earthquake of 1835, when
the linear north and south islet of St. Mary was uplifted several feet
above the surrounding area, we perhaps see one feeble step in the
formation of a subordinate mountain-axis. In some cases, moreover, for
instance, near the baths of Cauquenes, I was forcibly struck with the
small size of the breaches cut through the exterior mountain-ranges,
compared with the size of the same valleys higher up where entering the
Cordillera; and this circumstance appeared to me scarcely explicable,
except on the idea of the exterior lines having been subsequently
upthrown, and therefore having been exposed to a less amount of
denudation. From the manner in which the fringes of gravel are
prolonged in unbroken slopes up the valleys of the Cordillera, I infer
that most of the greater dislocations took place during the earlier
parts of the great elevation in mass: I have, however, elsewhere given
a case, and M. de Tschudi has given another, of a ridge thrown up in
Peru across the bed of a river, and consequently after the final
elevation of the country above the level of the sea. (“Reise in Peru”
Band 2 s.8: Author’s “Journal” 2nd edition page 359.)
Ascending to the older tertiary formations, I will not again
recapitulate the remarks already given at the end of the Fifth
Chapter,—on their great extent, especially along the shores of the
Atlantic—on their antiquity, perhaps corresponding with that of the
eocene deposits of Europe,—on the almost entire dissimilarity, though
the formations are apparently contemporaneous, of the fossils from the
eastern and western coasts, as is likewise the case, even in a still
more marked degree, with the shells now living in these opposite though
approximate seas,—on the climate of this period not having been more
tropical than what might have been expected from the latitudes of the
places under which the deposits occur; a circumstance rendered well
worthy of notice, from the contrast with what is known to have been the
case during the older tertiary periods of Europe, and likewise from the
fact of the southern hemisphere having suffered at a much later period,
apparently at the same time with the northern hemisphere, a colder or
more equable temperature, as shown by the zones formerly affected by
ice-action. Nor will I recapitulate the proofs of the bottom of the
sea, both on the eastern and western coast, having subsided seven or
eight hundred feet during this tertiary period; the movement having
apparently been co-extensive, or nearly co-extensive, with the deposits
of this age. Nor will I again give the facts and reasoning on which the
proposition was founded, that when the bed of the sea is either
stationary or rising, circumstances are far less favourable than when
its level is sinking, to the accumulation of conchiferous deposits of
sufficient thickness, extension, and hardness to resist, when upheaved,
the ordinary vast amount of denudation. We have seen that the highly
remarkable fact of the absence of any EXTENSIVE formations containing
recent shells, either on the eastern or western coasts of the
continent,—though these coasts now abound with living mollusca,—though
they are, and apparently have always been, as favourable for the
deposition of sediment as they were when the tertiary formations were
copiously deposited,—and though they have been upheaved to an amount
quite sufficient to bring up strata from the depths the most fertile
for animal life—can be explained in accordance with the above
proposition. As a deduction, it was also attempted to be shown, first,
that the want of close sequence in the fossils of successive
formations, and of successive stages in the same formation, would
follow from the improbability of the same area continuing slowly to
subside from one whole period to another, or even during a single
entire period; and secondly, that certain epochs having been favourable
at distant points, in the same quarter of the world for the synchronous
accumulation of fossiliferous strata, would follow from movements of
subsidence having apparently, like those of elevation,
contemporaneously affected very large areas.
There is another point which deserves some notice, namely, the analogy
between the upper parts of the Patagonian tertiary formation, as well
as of the upper possibly contemporaneous beds at Chiloe and Concepcion,
with the great gypseous formation of Cordillera; for in both
formations, the rocks, in their fusible nature, in their containing
gypsum, and in many other characters, show a connection, either
intimate or remote, with volcanic action; and as the strata in both
were accumulated during subsidence, it appears at first natural to
connect this sinking movement with a state of high activity in the
neighbouring volcanoes. During the cretaceo-oolitic period this
certainly appears to have been the case at the Puente del Inca, judging
from the number of intercalated lava-streams in the lower 3,000 feet of
strata; but generally, the volcanic orifices seem at this time to have
existed as submarine solfataras, and were certainly quiescent compared
with their state during the accumulation of the porphyritic
conglomerate formation. During the deposition of the tertiary strata we
know that at S. Cruz, deluges of basaltic lava were poured forth; but
as these lie in the upper part of the series, it is possible that the
subsidence may at that time have ceased: at Chiloe, I was unable to
ascertain to what part of the series the pile of lavas belonged. The
Uspallata tuffs and great streams of submarine lavas, were probably
intermediate in age between the cretaceo- oolitic and older tertiary
formations, and we know from the buried trees that there was a great
subsidence during their accumulation; but even in this case, the
subsidence may not have been strictly contemporaneous with the great
volcanic eruptions, for we must believe in at least one intercalated
period of elevation, during which the ground was upraised on which the
now buried trees grew. I have been led to make these remarks, and to
throw some doubt on the strict contemporaneousness of high volcanic
activity and movements of subsidence, from the conviction impressed on
my mind by the study of coral formations, that these two actions do not
generally go on synchronously;—on the contrary, that in volcanic
districts, subsidence ceases as soon as the orifices burst forth into
renewed action, and only recommences when they again have become
dormant. (“The Structure and Distribution of Coral Reefs.”)
At a later period, the Pampean mud, of estuary origin, was deposited
over a wide area,—in one district conformably on the underlying old
tertiary strata, and in another district unconformably on them, after
their upheaval and denudation. During and before the accumulation,
however, of these old tertiary strata, and, therefore, at a very remote
period, sediment, strikingly resembling that of the Pampas, was
deposited; showing during how long a time in this case the same
agencies were at work in the same area. The deposition of the Pampean
estuary mud was accompanied, at least in the southern parts of the
Pampas, by an elevatory movement, so that the M. Hermoso beds probably
were accumulated after the upheaval of those round the S. Ventana; and
those at P. Alta after the upheaval of the M. Hermoso strata; but there
is some reason to suspect that one period of subsidence intervened,
during which mud was deposited over the coarse sand of the Barrancas de
S. Gregorio, and on the higher parts of Banda Oriental. The mammiferous
animals characteristic of this formation, many of which differ as much
from the present inhabitants of South America, as do the eocene mammals
of Europe from the present ones of that quarter of the globe, certainly
co-existed at B. Blanca with twenty species of mollusca, one balanus,
and two corals, all now living in the adjoining sea: this is likewise
the case in Patagonia with the Macrauchenia, which co-existed with
eight shells, still the commonest kinds on that coast. I will not
repeat what I have elsewhere said, on the place of habitation, food,
wide range, and extinction of the numerous gigantic mammifers, which at
this late period inhabited the two Americas.
The nature and grouping of the shells embedded in the old tertiary
formations of Patagonia and Chile show us, that the continent at that
period must have stood only a few fathoms below its present level, and
that afterwards it subsided over a wide area, seven or eight hundred
feet. The manner in which it has since been rebrought up to its actual
level, was described in detail in the First and Second Chapters. It was
there shown that recent shells are found on the shores of the Atlantic,
from Tierra del Fuego northward for a space of at least 1,180 nautical
miles, and at the height of about 100 feet in La Plata, and of 400 feet
in Patagonia. The elevatory movements on this side of the continent
have been slow; and the coast of Patagonia, up to the height in one
part of 950 feet and in another of 1,200 feet, is modelled into eight
great, step-like, gravel-capped plains, extending for hundreds of miles
with the same heights; this fact shows that the periods of denudation
(which, judging from the amount of matter removed, must have been long
continued) and of elevation were synchronous over surprisingly great
lengths of coasts. On the shores of the Pacific, upraised shells of
recent species, generally, though not always, in the same proportional
numbers as in the adjoining sea, have actually been found over a north
and south space of 2,075 miles, and there is reason to believe that
they occur over a space of 2,480 miles. The elevation on this western
side of the continent has not been equable; at Valparaiso, within the
period during which upraised shells have remained undecayed on the
surface, it has been 1,300 feet, whilst at Coquimbo, 200 miles
northward, it has been within this same period only 252 feet. At Lima,
the land has been uplifted at least 80 feet since Indian man inhabited
that district; but the level within historical times apparently has
subsided. At Coquimbo, in a height of 364 feet, the elevation has been
interrupted by five periods of comparative rest. At several places the
land has been lately, or still is, rising both insensibly and by sudden
starts of a few feet during earthquake-shocks; this shows that these
two kinds of upward movement are intimately connected together. For a
space of 775 miles, upraised recent shells are found on the two
opposite sides of the continent; and in the southern half of this
space, it may be safely inferred from the slope of the land up to the
Cordillera, and from the shells found in the central part of Tierra del
Fuego, and high up the River Santa Cruz, that the entire breadth of the
continent has been uplifted. From the general occurrence on both coasts
of successive lines of escarpments, of sand-dunes and marks of erosion,
we must conclude that the elevatory movement has been normally
interrupted by periods, when the land either was stationary, or when it
rose at so slow a rate as not to resist the average denuding power of
the waves, or when it subsided. In the case of the present high
sea-cliffs of Patagonia and in other analogous instances, we have seen
that the difficulty in understanding how strata can be removed at those
depths under the sea, at which the currents and oscillations of the
water are depositing a smooth surface of mud, sand, and sifted pebbles,
leads to the suspicion that the formation or denudation of such cliffs
has been accompanied by a sinking movement.
In South America, everything has taken place on a grand scale, and all
geological phenomena are still in active operation. We know how violent
at the present day the earthquakes are, we have seen how great an area
is now rising, and the plains of tertiary origin are of vast
dimensions; an almost straight line can be drawn from Tierra del Fuego
for 1,600 miles northward, and probably for a much greater distance,
which shall intersect no formation older than the Patagonian deposits;
so equable has been the upheaval of the beds, that throughout this long
line, not a fault in the stratification or abrupt dislocation was
anywhere observable. Looking to the basal, metamorphic, and plutonic
rocks of the continent, the areas formed of them are likewise vast; and
their planes of cleavage and foliation strike over surprisingly great
spaces in uniform directions. The Cordillera, with its pinnacles here
and there rising upwards of twenty thousand feet above the level of the
sea, ranges in an unbroken line from Tierra del Fuego, apparently to
the Arctic circle. This grand range has suffered both the most violent
dislocations, and slow, though grand, upward and downward movements in
mass; I know not whether the spectacle of its immense valleys, with
mountain-masses of once liquified and intrusive rocks now bared and
intersected, or whether the view of those plains, composed of shingle
and sediment hence derived, which stretch to the borders of the
Atlantic Ocean, is best adapted to excite our astonishment at the
amount of wear and tear which these mountains have undergone.
The Cordillera from Tierra del Fuego to Mexico, is penetrated by
volcanic orifices, and those now in action are connected in great
trains. The intimate relation between their recent eruptions and the
slow elevation of the continent in mass, appears to me highly
important, for no explanation of the one phenomenon can be considered
as satisfactory which is not applicable to the other. (On the
Connection of certain Volcanic Phenomena in South America: “Geological
Transactions” volume 5 page 609.) The permanence of the volcanic action
on this chain of mountains is, also, a striking fact; first, we have
the deluges of submarine lavas alternating with the porphyritic
conglomerate strata, then occasionally feldspathic streams and abundant
mineral exhalations during the gypseous or cretaceo- oolitic period:
then the eruptions of the Uspallata range, and at an ancient but
unknown period, when the sea came up to the eastern foot of the
Cordillera, streams of basaltic lava at the foot of the Portillo range;
then the old tertiary eruptions; and lastly, there are here and there
amongst the mountains, much worn and apparently very ancient volcanic
formations without any craters; there are, also, craters quite extinct,
and others in the condition of solfataras, and others occasionally or
habitually in fierce action. Hence it would appear that the Cordillera
has been, probably with some quiescent periods, a source of volcanic
matter from an epoch anterior to our cretaceo-oolitic formation to the
present day; and now the earthquakes, daily recurrent on some part of
the western coast, give little hope that the subterranean energy is
expended.
Recurring to the evidence by which it was shown that some at least of
the parallel ridges, which together compose the Cordillera, were
successively and slowly upthrown at widely different periods; and that
the whole range certainly once, and almost certainly twice, subsided
some thousand feet, and being then brought up by a slow movement in
mass, again, during the old tertiary formations, subsided several
hundred feet, and again was brought up to its present level by a slow
and often interrupted movement; we see how opposed is this complicated
history of changes slowly effected, to the views of those geologists
who believe that this great mountain-chain was formed in late times by
a single blow. I have endeavoured elsewhere to show, that the
excessively disturbed condition of the strata in the Cordillera, so far
from indicating single periods of extreme violence, presents
insuperable difficulties, except on the admission that the masses of
once liquified rocks of the axes were repeatedly injected with
intervals sufficiently long for their successive cooling and
consolidation. (“Geological Transactions” volume 5 page 626.) Finally,
if we look to the analogies drawn from the changes now in progress in
the earth’s crust, whether to the manner in which volcanic matter is
erupted, or to the manner in which the land is historically known to
have risen and sunk: or again, if we look to the vast amount of
denudation which every part of the Cordillera has obviously suffered,
the changes through which it has been brought into its present
condition, will appear neither to have been too slowly effected, nor to
have been too complicated.
NOTE.
As, both in France and England, translations of a passage in Professor
Ehrenberg’s Memoir, often referred to in the Fourth Chapter of this
volume, have appeared, implying that Professor Ehrenberg believes, from
the character of the infusoria, that the Pampean formation was
deposited by a sea-debacle rushing over the land, I may state, on the
authority of a letter to me, that these translations are incorrect. The
following is the passage in question:—
“Durch Beachtung der mikroscopischen Formen hat sich nun feststellen
lassen, das die Mastodonten-Lager am La Plata und die Knochen-Lager am
Monte Hermoso, who wie die der Riesen-Gurtelthiere in den Dunenhugeln
bei Bahia Blanca, beides in Patagonien, unveranderte brakische
Susswasserbildungen sind, die einst wohl sammtlich zum obersten
Fluthgebiethe des Meeres im tieferen Festlande
gehorten.”—“Monatsberichten der konigl. Akad. etc.” zu Berlin vom April
1845.
INDEX.
Abich, on a new variety of feldspar.
Abrolhos islands.
Absence of recent formations on the S. American coasts.
Aguerros on elevation of Imperial.
Albite, constituent mineral in andesite. —in rocks of Tierra del Fuego.
—in porphyries. —crystals of, with orthite.
Alison, Mr., on elevation of Valparaiso.
Alumina, sulphate of.
Ammonites from Concepcion.
Amolanas, Las.
Amygdaloid, curious varieties of.
Amygdaloids of the Uspallata range. —of Copiapo.
Andesite of Chile. —in the valley of Maypu. —of the Cumbre pass. —of
the Uspallata range. —of Los Hornos. —of Copiapo.
Anhydrite, concretions of.
Araucaria, silicified wood of. Arica, elevation of.
Arqueros, mines of.
Ascension, gypsum deposited on. —laminated volcanic rocks of.
Augite in fragments, in gneiss. —with albite, in lava.
Austin, Mr. R.A.C., on bent cleavage lamina.
Austin, Captain, on sea-bottom.
Australia, foliated rocks of.
Azara labiata, beds of, at San Pedro.
Baculites vagina.
Bahia Blanca, elevation of. —formations near. —character of living
shells of.
Bahia (Brazil), elevation near. —crystalline rocks of.
Ballard, M., on the precipitation of sulphate of soda.
Banda Oriental, tertiary formations of. —crystalline rocks of.
Barnacles above sea-level. —adhering to upraised shells.
Basalt of S. Cruz. —streams of, in the Portillo range. —in the
Uspallata range.
Basin chains of Chile.
Beagle Channel.
Beaumont, Elie de, on inclination of lava-streams. —on viscid
quartz-rocks.
Beech-tree, leaves of fossil.
Beechey, Captain, on sea-bottom.
Belcher, Lieutenant, on elevated shells from Concepcion.
Bella Vista, plain of.
Benza, Dr., on decomposed granite.
Bettington, Mr., on quadrupeds transported by rivers.
Blake, Mr., on the decay of elevated shells near Iquique. —on nitrate
of soda.
Bole.
Bollaert, Mr., on mines of Iquique.
Bones, silicified. —fossil, fresh condition of.
Bottom of sea off Patagonia.
Bougainville, on elevation of the Falkland islands.
Boulder formation of S. Cruz. —of Falkland islands. —anterior to
certain extinct quadrupeds. —of Tierra del Fuego.
Boulders in the Cordillera. —transported by earthquake-waves. —in
fine-grained tertiary deposits.
Brande, Mr., on a mineral spring.
Bravais, M., on elevation of Scandinavia.
Brazil, elevation of. —crystalline rocks of.
Broderip, Mr., on elevated shells from Concepcion.
Brown, Mr. R., on silicified wood of Uspallata range.
Brown, on silicified wood.
Bucalema, elevated shells near.
Buch, Von, on cleavage. —on cretaceous fossils of the Cordillera. —on
the sulphureous volcanoes of Java.
Buenos Ayres.
Burchell, Mr., on elevated shells of Brazil.
Byron, on elevated shells.
Cachapual, boulders in valley of.
Caldcleugh, Mr., on elevation of Coquimbo. —on rocks of the Portillo
range.
Callao, elevation near. —old town of.
Cape of Good Hope, metamorphic rocks of.
Carcharias megalodon.
Carpenter, Dr., on microscopic organisms.
Castro (Chiloe), beds near.
Cauquenes Baths, boulders near. —pebbles in porphyry near. —volcanic
formation near. —stratification near.
Caves above sea-level.
Cervus pumilus, fossil-horns of.
Chevalier, M., on elevation near Lima.
Chile, structure of country between the Cordillera and the Pacific.
—tertiary formations of. —crystalline rocks in. —central, geology of.
—northern, geology of.
Chiloe, gravel on coast. —elevation of. —tertiary formation of.
—crystalline rocks of.
Chlorite-schist, near M. Video.
Chonos archipelago, tertiary formations of. —crystalline rocks of.
Chupat, Rio, scoriae transported by.
Claro, Rio, fossiliferous beds of.
Clay-shale of Los Hornos.
Clay-slate, formation of, Tierra del Fuego. —of Concepcion.
—feldspathic, of Chile. — —of the Uspallata range. —black siliceous,
band of, in porphyritic formations of Chile.
Claystone porphyry, formation of, in Chile. —origin of. —eruptive
sources of.
Cleavage, definition of. —at Bahia. —Rio de Janeiro. —Maldonado. —Monte
Video. —S. Guitru-gueyu. —Falkland I. —Tierra del Fuego. —Chonos I.
—Chiloe. —Concepcion. —Chile. —discussion on.
Cleavage-laminae superficially bent.
Cliffs, formation of.
Climate, late changes in. —of Chile during tertiary period.
Coal of Concepcion. —S. Lorenzo.
Coast-denudation of St. Helena.
Cobija, elevation of.
Colombia, cretaceous formation of.
Colonia del Sacramiento, elevation of. —Pampean formation near
Colorado, Rio, gravel of. —sand-dunes of. —Pampean formation near.
Combarbala.
Concepcion, elevation of. —deposits of. —crystalline rocks of.
Conchalee, gravel-terraces of.
Concretions of gypsum, at Iquique. —in sandstone at S. Cruz. —in
tufaceous tuff of Chiloe. —in gneiss. —in claystone-porphyry at Port
Desire. —in gneiss at Valparaiso. —in metamorphic rocks. —of anhydrite.
—relations of, to veins.
Conglomerate claystone of Chile. —of Tenuyan. —of the Cumbre Pass. —of
Rio Claro. —of Copiapo.
Cook, Captain, on form of sea-bottom.
Copiapo, elevation of. —tertiary formations of. —secondary formations
of.
Copper, sulphate of. —native, at Arqueros. —mines of, at Panuncillo.
—veins, distribution of.
Coquimbo, elevation and terraces of. —tertiary formations of.
—secondary formations of.
Corallines living on pebbles.
Cordillera, valleys bordered by gravel fringes. —basal strata of.
—fossils of. —elevation of. —gypseous formations of.
—claystone-porphyries of. —andesitic rocks of. —volcanoes of.
Coste, M., on elevation of Lemus.
Coy inlet, tertiary formation of.
Crassatella Lyellii.
Cruickshanks, Mr., on elevation near Lima.
Crystals of feldspar, gradual formation of, at Port Desire.
Cumbre, Pass of, in Cordillera.
Cuming, Mr., on habits of the Mesodesma. —on range of living shells on
west coast.
Dana, Mr., on foliated rocks. —on amygdaloids.
Darwin, Mount.
D’Aubuisson, on concretions. —on foliated rocks. Decay, gradual, of
upraised shells.
Decomposition of granite rocks.
De la Beche, Sir H., his theoretical researches in geology. —on the
action of salt on calcareous rocks. —on bent cleavage-laminae.
Denudation on coast of Patagonia. —great powers of. —of the Portillo
range.
Deposits, saline.
Despoblado, valley of.
Detritus, nature of, in Cordillera.
Devonshire, bent cleavage in.
Dikes, in gneiss of Brazil. —near Rio de Janeiro. —pseudo, at Port
Desire. —in Tierra del Fuego. —in Chonos archipelago, containing
quartz. —near Concepcion, with quartz. —granitic-porphyritic, at
Valparaiso. —rarely vesicular in Cordillera. —absent in the central
ridges of the Portillo pass. —of the Portillo range, with grains of
quartz. —intersecting each other often. —numerous at Copiapo.
Domeyko, M., on the silver mines of Coquimbo. on the fossils of
Coquimbo.
D’Orbigny, M. A., on upraised shells of Monte Video. —on elevated
shells at St. Pedro. —on elevated shells near B. Ayres. —on elevation
of S. Blas. —on the sudden elevation of La Plata. —on elevated shells
near Cobija. —on elevated shells near Arica. —on the climate of Peru.
—on salt deposits of Cobija. —on crystals of gypsum in salt-lakes. —on
absence of gypsum in the Pampean formation. —on fossil remains from
Bahia Blanca. —on fossil remains from the banks of the Parana. —on the
geology of St. Fe. —on the age of Pampean formation. —on the Mastodon
Andium. —on the geology of the Rio Negro. —on the character of the
Patagonian fossils. —on fossils from Concepcion. — —from Coquimbo. —
—from Payta. —on fossil tertiary shells of Chile. —on cretaceous
fossils of Tierra del Fuego. — —from the Cordillera of Chile.
Earth, marine origin of.
Earthenware, fossil.
Earthquake, effect of, at S. Maria. —elevation during, at Lemus. —of
1822, at Valparaiso. —effects of, in shattering surface. —fissures made
by. —probable effects on cleavage.
Earthquakes in Pampas.
Earthquake-waves, power of, in throwing up shells. —effects of, near
Lima. —power of, in transporting boulders.
Edmonston, Mr., on depths at which shells live at Valparaiso.
Ehrenberg, Professor, on infusoria in the Pampean formation. —on
infusoria in the Patagonian formation.
Elevation of La Plata. —Brazil. —Bahia Blanca. —San Blas. —Patagonia.
—Tierra del Fuego. —Falkland islands. —Pampas. —Chonos archipelago.
—Chiloe. —Chile. —Valparaiso. —Coquimbo. —Guasco. —Iquique. —Cobija.
—Lima. —sudden, at S. Maria. — —at Lemus. —insensible, at Chiloe. — —at
Valparaiso. — —at Coquimbo. —axes of, at Chiloe. — —at P. Rumena. —at
Concepcion. —unfavourable for the accumulation of permanent deposits.
—lines of, parallel to cleavage and foliation. —lines of, oblique to
foliation. —areas of, causing lines of elevation and cleavage. —lines
of, in the Cordillera. —slow, in the Portillo range. —two periods of,
in Cordillera of Central Chile. —of the Uspallata range. —two periods
of, in Cumbre Pass. —horizontal, in the Cordillera of Copiapo. —axes
of, coincident with volcanic orifices. —of the Cordillera, summary on.
Elliott, Captain, on human remains.
Ensenada, elevated shells of.
Entre Rios, geology of.
Equus curvidens.
Epidote in Tierra del Fuego. —in gneiss. —frequent in Chile. —in the
Uspallata range. —in porphyry of Coquimbo.
Erman, M., on andesite. Escarpments, recent, of Patagonia.
Extinction of fossil mammifers.
Falkland islands, elevation of. —pebbles on coast. —geology of.
Falkner, on saline incrustations.
Faults, great, in Cordillera.
Feldspar, earthy, metamorphosis of, at Port Desire. —albitic. —crystals
of, with albite. —orthitic, in conglomerate of Tenuyan. —in granite of
Portillo range. —in porphyries in the Cumbre Pass.
Feuillee on sea-level at Coquimbo.
Fissures, relations of, to concretions. —upfilled, at Port Desire. —in
clay-slate.
Fitton, Dr., on the geology of Tierra del Fuego.
Fitzroy, Captain, on the elevation of the Falkland islands. —on the
elevation of Concepcion.
Foliation, definition of. —of rocks at Bahia. —Rio de Janeiro.
—Maldonado. —Monte Video. —S. Guitru-gueyu. —Falkland I. —Tierra del
Fuego. —Chonos archipelago. —Chiloe. —Concepcion. —Chile. —discussion
on.
Forbes, Professor E., on cretaceous fossils of Concepcion. —on
cretaceous fossils and subsidence in Cumbre Pass. —on fossils from
Guasco. — —from Coquimbo. — —from Copiapo. —on depths at which shells
live.
Formation, Pampean. — —area of. — —estuary origin. —tertiary of Entre
Rios. —of Banda Oriental. —volcanic, in Banda Oriental. —of Patagonia.
—summary on. —tertiary of Tierra del Fuego. — —of the Chonos
archipelago. — —of Chiloe. — —of Chile. — —of Concepcion. — —of
Navidad. — —of Coquimbo. — —of Peru. — —subsidence during. —volcanic,
of Tres Montes. — —of Chiloe. — —old, near Maldonado. — —with laminar
structure. — —ancient, in Tierra del Fuego. —recent, absent on S.
American coast. —metamorphic, of claystone-porphyry of Patagonia.
—foliation of. —plutonic, with laminar structure. —palaeozoic, of the
Falkland I. —claystone, at Concepcion. —Jurassic, of Cordillera.
—Neocomian, of the Portillo Pass. —volcanic, of Cumbre Pass. —gypseous,
of Los Hornos. — —of Coquimbo. — —of Guasco. — —of Copiapo. — —of
Iquique. —cretaceo-oolitic, of Coquimbo. — —of Guasco. — —of Copiapo. —
—of Iquique.
Fossils, Neocomian, of Portillo Pass. — —of Cumbre Pass. —secondary, of
Coquimbo. — —of Guasco. — —of Copiapo. — —of Iquique. —palaeozoic, from
the Falklands.
Fragments of hornblende-rock in gneiss. —of gneiss in gneiss.
Freyer, Lieutenant, on elevated shells of Arica.
Frezier on sea-level at Coquimbo.
Galapagos archipelago, pseudo-dikes of.
Gallegos, Port, tertiary formation of.
Garnets in gneiss. —in mica-slate. —at Panuncillo.
Gardichaud, M., on granites of Brazil.
Gay, M., on elevated shells. —on boulders in the Cordillera. —on
fossils from Cordillera of Coquimbo.
Gill, Mr., on brickwork transported by an earthquake-wave.
Gillies, Dr., on heights in the Cordillera. —on extension of the
Portillo range.
Glen Roy, parallel roads of. —sloping terraces of.
Gneiss, near Bahia. —of Rio de Janeiro. —decomposition of.
Gold, distribution of.
Gorodona, formations near. Granite, axis of oblique, to foliation.
—andesitic. —of Portillo range. —veins of, quartzose. —pebble of, in
porphyritic conglomerate. —conglomerate.
Grauwacke of Uspallata range.
Gravel at bottom of sea. —formation of, in Patagonia. —means of
transportation of. —strata of, inclined.
Gravel-terraces in Cordillera.
Greenough, Mr., on quartz veins.
Greenstone, resulting from metamorphose hornblende-rock. —of Tierra del
Fuego. —on the summit of the Campana of Quillota. —porphyry. —relation
of, to clay-slate.
Gryphaea orientalis.
Guasco, elevation of. —secondary formation of.
Guitru-gueyu, Sierra.
Guyana, gneissic rocks of.
Gypsum, nodules of, in gravel at Rio Negro. —deposited from sea-water.
—deposits of, at Iquique. —crystals of, in salt lakes. —in Pampean
formation. —in tertiary formation of Patagonia. —great formation of, in
the Portillo Pass. — —in the Cumbre Pass. — —near Los Hornos. — —at
Coquimbo. — —at Copiapo. — —near Iquique. —of San Lorenzo.
Hall, Captain, on terraces at Coquimbo.
Hamilton, Mr., on elevation near Tacna.
Harlan, Dr., on human remains.
Hayes, Mr. A., on nitrate of soda.
Henslow, Professor, on concretions.
Herbert, Captain, on valleys in the Himalaya.
Herradura Bay, elevated shells of. —tertiary formations of.
Himalaya, valleys in.
Hippurites Chilensis.
Hitchcock, Professor, on dikes.
Honestones, pseudo, of Coquimbo. —of Copiapo.
Hooker, Dr. J.D., on fossil beech-leaves.
Hopkins, Mr., on axes of elevation oblique to foliation. —on origin of
lines of elevation.
Hornblende-rock, fragments of, in gneiss.
Hornblende-schist, near M. Video.
Hornos, Los, section near.
Hornstone, dike of.
Horse, fossil tooth of.
Huafo island. —subsidence at.
Huantajaya, mines of.
Humboldt, on saline incrustations. —on foliations of gneiss. —on
concretions in gneiss.
Icebergs, action on cleavage.
Illapel, section near.
Imperial, beds of shells near.
Incrustations, saline.
Infusoria in Pampean formation. —in Patagonian formation.
Iodine, salts of.
Iquique, elevation of. —saliferous deposits of. —cretaceo-oolitic
formation of.
Iron, oxide of, in lavas. —in sedimentary beds. —tendency in, to
produce hollow concretions. —sulphate of.
Isabelle, M., on volcanic rocks of Banda Oriental.
Joints in clay-slate.
Jukes, Mr., on cleavage in Newfoundland.
Kamtschatka, andesite of.
Kane, Dr., on the production of carbonate of soda.
King George’s sound, calcareous beds of.
Lakes, origin of. —fresh-water, near salt lakes.
Lava, basaltic, of S. Cruz. —claystone-porphyry, at Chiloe. — —ancient
submarine. —basaltic, of the Portillo range. —feldspathic, of the
Cumbre Pass. —submarine, of the Uspallata range. —basaltic, of the
Uspallata range. —submarine, of Coquimbo. —of Copiapo.
Lemus island.
Lemuy islet.
Lignite of Chiloe. —of Concepcion.
Lima, elevation of.
Lime, muriate of.
Limestone of Cumbre Pass. —of Coquimbo. —of Copiapo.
Lund and Clausen on remains of caves in Brazil.
Lund, M., on granites of Brazil.
Lyell, M., on upraised shells retaining their colours. —on terraces at
Coquimbo. —on elevation near Lima. —on fossil horse’s tooth. —on the
boulder-formation being anterior to the extinction of North American
mammifers. —on quadrupeds washed down by floods. —on age of American
fossil mammifers. —on changes of climate. —on denudation. —on
foliation.
MacCulloch, Dr., on concretions. —on beds of marble.
Maclaren, Mr., letter to, on coral-formations.
Macrauchenia Patachonica.
Madeira, subsidence of.
Magellan, Strait, elevation near, of.
Magnesia, sulphate of, in veins.
Malcolmson, Dr., on trees carried out to sea.
Maldonado, elevation of. —Pampean formation of. —crystalline rocks of.
Mammalia, fossil, of Bahia Blanca. — —near St. Fe. — —of Banda
Oriental. — —of St. Julian. — —at Port Gallegos. —washed down by
floods. —number of remains of, and range of, in Pampas.
Man, skeletons of (Brazil). —remains of, near Lima. —Indian, antiquity
of.
Marble, beds of.
Maricongo, ravine of.
Marsden, on elevation of Sumatra.
Mastodon Andium, remains of. —range of.
Maypu, Rio, mouth of, with upraised shells. —gravel fringes of.
—debouchement from the Cordillera.
Megalonyx, range of.
Megatherium, range of.
Miers, Mr., on elevated shells. —on the height of the Uspallata plain.
Minas, Las.
Mocha Island, elevation of. —tertiary form of. —subsidence at.
Molina, on a great flood.
Monte Hermoso, elevation of. —fossils of.
Monte Video, elevation of. —Pampean formation of. —crystalline rocks
of.
Morris and Sharpe, Messrs., on the palaeozoic fossils of the Falklands.
Mud, Pampean. —long deposited on the same area.
Murchison, Sir R., on cleavage. —on waves transporting gravel. —on
origin of salt formations. —on the relations of metalliferous veins and
intrusive rocks. —on the absence of granite in the Ural.
Nautilus d’Orbignyanus.
Navidad, tertiary formations of, subsidence of.
Negro, Rio, pumice of pebbles of. —gravel of. —salt lakes of. —tertiary
strata of.
North America, fossil remains of.
North Wales, sloping terraces absent in. —bent cleavage of.
Neuvo Gulf, plains of. —tertiary formation of.
Owen, Professor, on fossil mammiferous remains.
Palmer, Mr., on transportation of gravel.
Pampas, elevation of. —earthquakes of. —formation of. —localities in
which fossil mammifers have been found.
Panuncillo, mines of.
Parana, Rio, on saline incrustations. —Pampean formations near. —on the
S. Tandil.
Parish, Sir W., on elevated shells near Buenos Ayres. —on earthquakes
in the Pampas. —on fresh-water near salt lakes. —on origin of Pampean
formation.
Patagonia, elevation and plains of. —denudation of. —gravel-formation
of. —sea-cliffs of. —subsidence during tertiary period. —crystalline
rocks of.
Payta, tertiary formations of.
Pebbles of pumice. —decrease in size on the coast of Patagonia. —means
of transportation. —encrusted with living corallines. —distribution of,
at the eastern foot of Cordillera. —dispersal of, in the Pampas. —zoned
with colour.
Pentland, Mr., on heights in the Cordillera. —on fossils of the
Cordillera.
Pernambuco.
Peru, tertiary formations of.
Peuquenes, Pass of, in the Cordillera. —ridge of.
Pholas, elevated shells of.
Pitchstone of Chiloe. —of Port Desire. —near Cauquenes. —layers of, in
the Uspallata range. —of Los Hornos. —of Coquimbo.
Plains of Patagonia. —of Chiloe. —of Chile. —of Uspallata. —on eastern
foot of Cordillera. —of Iquique.
Plata, La, elevation of. —tertiary formation of. —crystalline rocks of.
Playfair, Professor, on the transportation of gravel.
Pluclaro, axis of.
Pondicherry, fossils of.
Porcelain rocks of Port Desire. —of the Uspallata range.
Porphyry, pebbles of, strewed over Patagonia.
Porphyry, claystone, of Chiloe, — —of Patagonia. — —of Chile.
—greenstone, of Chile. —doubly columnar. —claystone, rare, on the
eastern side of the Portillo Pass. —brick-red and orthitic, of Cumbre
Pass. —intrusive, repeatedly injected. —claystone of the Uspallata
range. — —of Copiapo. — —eruptive sources of.
Port Desire, elevation and plains of. —tertiary formation of.
—porphyries of.
Portillo Pass in the Cordillera.
Portillo chain. —compared with that of the Uspallata.
Prefil or sea-wall of Valparaiso.
Puente del Inca, section of.
Pumice, pebbles of. —conglomerate of R. Negro. —hills of, in the
Cordillera.
Punta Alta, elevation of. —beds of.
Quartz-rock of the S. Ventana. —C. Blanco. —Falkland islands. —Portillo
range. —viscidity of. —veins of, near Monte Video. — —in dike of
greenstone. —grains of, in mica slate. — —in dikes. —veins of,
relations to cleavage.
Quillota, Campana of.
Quintero, elevation of.
Quiriquina, elevation of. —deposits of.
Rancagua, plain of.
Rapel, R. elevation near.
Reeks, Mr. T., his analysis of decomposed shells. —his analysis of
salts.
Remains, human.
Rio de Janeiro, elevation near. —crystalline rocks of.
Rivers, small power of transporting pebbles. —small power of, in
forming valleys. —drainage of, in the Cordillera.
Roads, parallel, of Glen Roy.
Rocks, volcanic, of Banda Oriental. —Tres Montes. —Chiloe. —Tierra del
Fuego. —with laminar structure.
Rodents, fossil, remains of.
Rogers, Professor, address to Association of American Geologists.
Rose, Professor G., on sulphate of iron at Copiapo.
S. Blas, elevation of.
S. Cruz, elevation and plains of. —valley of. —nature of gravel in
valley of. —boulder formation of. —tertiary formation of. —subsidence
at.
S. Fe Bajada, formations of.
S. George’s bay, plains of.
S. Helena island, sea-cliffs, and subsidence of.
S. Josef, elevation of. —tertiary formation of.
S. Juan, elevation near.
S. Julian, elevation and plains of. —salt lake of. —earthy deposit with
mammiferous remains. —tertiary formations of. —subsidence at.
S. Lorenzo, elevation of. —old salt formation of.
S. Mary, island of, elevation of.
S. Pedro, elevation of.
Salado, R., elevated shells of. —Pampean formation of.
Salines.
Salt, with upraised shell. —lakes of. —purity of, in salt lakes.
—deliquescent, necessary for the preservation of meat. —ancient
formation of, at Iquique. — —at S. Lorenzo. —strata of, origin of.
Salts, superficial deposits of.
Sand-dunes of the Uruguay. —of the Pampas. —near Bahia Blanca. —of the
Colorado. —of S. Cruz. —of Arica.
Sarmiento, Mount.
Schmidtmeyer on auriferous detritus.
Schomburghk, Sir R., on sea-bottom. —on the rocks of Guyana.
Scotland, sloping terraces of.
Sea, nature of bottom of, off Patagonia. —power of, in forming valleys.
Sea cliffs, formation of.
Seale, Mr., model of St. Helena.
Sebastian Bay, tertiary formation of.
Sedgwick, Professor, on cleavage.
Serpentine of Copiapo.
Serpulae, on upraised rocks.
Shale-rock, of the Portillo Pass. —of Copiapo.
Shells, upraised state of, in Patagonia. —elevated, too small for human
food. —transported far inland, for food. —upraised, proportional
numbers varying. — —gradual decay of. — —absent on high plains of
Chile. — —near Bahia Blanca. —preserved in concretions. —living and
fossil range of, on west coast. —living, different on the east and west
coast.
Shingle of Patagonia.
Siau, M., on sea-bottom.
Silver mines of Arqueros. —of Chanuncillo. —of Iquique. —distribution
of.
Slip, great, at S. Cruz.
Smith, Mr., of Jordan Hill, on upraised shells retaining their colours.
—on Madeira. —on elevated seaweed. —on inclined gravel beds.
Soda, nitrate of. —sulphate of, near Bahia Blanca. —carbonate of.
Soundings off Patagonia. —in Tierra del Fuego.
Spirifers.
Spix and Martius on Brazil. Sprengel on the production of carbonate of
soda.
Springs, mineral, in the Cumbre Pass.
Stratification of sandstone in metamorphic rocks. —of clay-slate in
Tierra del Fuego. —of the Cordillera of Central Chile. —little
disturbed in Cumbre Pass. —disturbance of, near Copiapo.
Streams of lava at S. Cruz, inclination of. —in the Portillo range.
String of cotton with fossil-shells.
Struthiolaria ornata.
Studer, M., on metamorphic rocks.
Subsidence during formation of sea-cliffs. —near Lima. —probable,
during Pampean formation. —necessary for the accumulation of permanent
deposits. —during the tertiary formations of Chile and Patagonia.
—probable during the Neocomian formation of the Portillo Pass.
—probable during the formation of conglomerate of Tenuyan. —during the
Neocomian formation of the Cumbre Pass. —of the Uspallata range.
—great, at Copiapo. — —during the formation of the Cordillera.
Sulphur, volcanic exhalations of.
Sumatra, promontories of.
A Summary on the recent elevatory movements. —on the Pampean formation.
—on the tertiary formations of Patagonia and Chile. —on the Chilean
Cordillera. —on the cretaceo-oolitic formation. —on the subsidences of
the Cordillera. —on the elevation of the Cordillera.
Tacna, elevation of.
Tampico, elevated shells near.
Tandil, crystalline rocks of.
Tapalguen, Pampean formation of. —crystalline rocks of.
Taylor, Mr., on copper veins of Cuba.
Temperature of Chile during the tertiary period.
Tension, lines of, origin of, axes of elevation and of cleavage.
Tenuy Point, singular section of.
Tenuyan, valley of.
Terraces of the valley of S. Cruz. —of equable heights throughout
Patagonia. —of Patagonia, formation of. —of Chiloe. —at Conchalee. —of
Coquimbo. —not horizontal at Coquimbo. —of Guasco. —of S. Lorenzo. —of
gravel within the Cordillera.
Theories on the origin of the Pampean formation.
Tierra Amarilla.
Tierra del Fuego, form of sea-bottom. —tertiary formations of.
—clay-slate formation of. —cretaceous formation of. —crystalline rocks
of. —cleavage of clay-slate.
Tosca rock.
Trachyte of Chiloe. —of Port Desire. —in the Cordillera.
Traditions of promontories having been islands. —on changes of level
near Lima.
Trees buried in plain of Iquique. —silicified, vertical, of the
Uspallata range.
Tres Montes, elevation of. —volcanic rocks of.
Trigonocelia insolita.
Tristan Arroyo, elevated shells of.
Tschudi, Mr., on subsidence near Lima.
Tuff, calcareous, at Coquimbo. —on basin-plain near St. Jago.
—structure of, in Pampas. —origin of, in Pampas. —pumiceous, of R.
Negro. —Nuevo Gulf. —Port Desire. —S. Cruz. —Patagonia, summary on
Chiloe. —formation of, in Portillo chain. —great deposit of, at
Copiapo.
Tuffs, volcanic, metamorphic, of Uspallata. —of Coquimbo.
Ulloa, on rain in Peru. —on elevation near Lima.
Uruguay, Rio, elevation of country near.
Uspallata, plain of. —pass of. —range of. —concluding remarks on.
Valdivia, tertiary beds of. —mica-slate of.
Valley of S. Cruz, structure of. —Coquimbo. —Guasco, structure of.
—Copiapo, structure of. —S. Cruz, tertiary formations of. —Coquimbo,
geology of. —Guasco, secondary formations of. —Copiapo, secondary
formations of. —Despoblado.
Valleys in the Cordillera bordered by gravel fringes. —formation of.
—in the Cordillera.
Valparaiso, elevation of. —gneiss of.
Vein of quartz near Monte Video. —in mica-slate. —relations of, to
cleavage. —in a trap dike. —of granite, quartzose. —remarkable, in
gneiss, near Valparaiso.
Veins, relations of, to concretions. —metalliferous, of the Uspallata
range. —metalliferous, discussion on.
Venezuela, gneissic rocks of.
Ventana, Sierra, Pampean formation near. —quartz-rock of.
Villa Vincencio Pass.
Volcan, Rio, mouth of. —fossils of.
Volcanoes of the Cordillera. —absent, except near bodies of water.
—ancient submarine, in Cordillera. —action of, in relation to changes
of level. —long action of, in the Cordillera.
Wafer on elevated shells.
Waves caused by earthquakes, power of, in transporting boulders. —power
of, in throwing up shells.
Weaver, Mr., on elevated shells.
White, Martin., on sea-bottom.
Wood, silicified, of Entre Rios. —S. Cruz. —Chiloe. —Uspallata range.
—Los Hornos. —Copiapo.
Yeso, Rio, and plain of.
Ypun Island, tertiary formation of.
Zeagonite.
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